During the Vietnam War, the Soviets sent hundreds of MiGs to North Vietnam. Now, Americans say they cannot send airplanes to Ukraine lest it lead to escalation.
Why wasn't the Soviet assistance to North Vietnam during the Vietnam War a form of escalation? Why didn't it start the 3rd World War, say, via an American attack on the USSR as revenge for the latter's direct involvement in the Vietnam War?
As with many things about international relations, the issue is not so much if they can do it but if it is the convenient (for oneself, not for Ukraine) thing to do.
To top your analogy, the USA did send planes, ships, weapons and half a million men to the Vietnam War without seeming to be particularly worried about nuclear retaliation from the Soviet Union.
But the Vietnam War always was a proxy war: both sides wanted to win it, of course, but neither would have felt particularly threatened by losing it. Whoever would lose it, would still be safe relying on its nuclear arsenal, its other military and political assets, and the distance from Vietnam to their core interests.
In Ukraine, Russia has repeatedly asserted that it views it as an existential threat if they do not achieve its objectives or if the West intervenes militarily. That means believing that if they lose the war, they will lose not only Ukrania but Russia itself (or perhaps more properly, its government) would be at risk of being destroyed.
This could be a bluff: Putin playing the madman to scare the West. Or it could be his actual assessment, and in this case the risk of losing the war could outweigh the risk of using nuclear weapons.
It is difficult to be sure which is the case, but in any case it has made the West way more cautious than they have been in other situations (e.g. support to Syrian or Libyan rebels).
The stakes aren't that high.
Vietnam was waged on Domino Theory which was influential at the time: basically, if Vietnam fell, then SE Asia would fall. This anxiety is what led the US to engage in a long war that was hard to win, saw the US mull over nuclear options a few times and led to severely drawing down US troops level in Western Europe.
At the time, the Warsaw Pact and NATO both were locked in an existential struggle for world dominance, to politically and conventionally annihilate the opposite system if at all possible. Nuclear weapons were a constant reminder to stay out of escalatory situations, such as NATO on Warsaw combat, but otherwise, almost every dirty trick was good enough. The stakes were high enough that skirting nuclear risks was acceptable at times. After all, a gradual nibbling down of NATO allies and neutral countries would make the Warsaw Pact stronger.
Not least because Communism, like it or not, was an ideology that was appealing to many, so it could self-sustain by absorbing new gains.
Contrast Ukraine in 2022. Ukraine deserves its freedom, certainly. Its people deserve to live in peace and freedom and they deserve the join the political clubs that they wish to be part of (even if NATO membership is too provocative to Russia). They don't deserve to be bullied and bombed by their neighbor.
But if Ukraine falls, it will, far from strengthening Russia, merely weaken it by forcing to occupy. The Ukrainian people have seen the results of the Afghanistan and Vietnam wars, they know time would be on their side there as well.
From the West's viewpoint, not necessarily the viewpoint of Ukrainians doing the dying, things need to be kept in context. For example, Wesley Clark (retired NATO chief) on CNN was saying that, if we don't stop Putin now, then we will be fighting Russian in Poland later. Which he then segued into advocating for the no-fly-zone. Given Russia's performance to date in Ukraine, the idea that they could achieve much attacking NATO directly is a fantasy. Even if Russia had performed much better, they just don't have the industrial capacity to challenge NATO past the opening weeks of hostilities. So, yes, this type of talk is a dangerous fantasy of "act now, regardless of the risks, because 1938 Czechoslovakia". There are parallels but Western states have, so far, risen far above Chamberlain's model.
That's what I mean by the stakes not being that high. They don't justify ratcheting up nuclear risks, and especially not for half-assed ideas that would be of little actual benefit.
Also, while we should not be looking for ways to placate Putin, we should also not provide him with ready-made propaganda justifying his actions to the Russian people. Who have all sorts of reasons to gradually turn on him if they don't feel their country is actually threatened and start seeing the downsides of this war.
We ignored Putin's nature for far too long, but let's not build him into a behemoth either. Even if Russia had taken over in a quick surgical strike decapitating the elected Ukrainian government it still has nowhere the same ratio in conventional forces against NATO than it did in the 70s and 80s.
Nor does its ideology of kleptocracy export particularly well. The oligarchs may buy property and spread corruption around but no mass popular movement is going to arise anywhere saying "I know, let's just run our country like Russia does it". Some did with Communism, some do with Fundamentalist Islam, no people will want to consciously emulate the Russian model however (though their elites might).
So we must not make the cure worse than the disease. The long term risk from Russia, using conventional weapons, is low.
However both sides are still full-fledged nuclear powers, so it behooves our leaders to be very, very cautious in what they consider escalatory or not. Jet fighters would not help that much, but they could easily up the ante.
FWIW, ISW's March 19th assessment is that
Ukrainian forces have defeated the initial Russian campaign of this war. That campaign aimed to conduct airborne and mechanized operations to seize Kyiv, Kharkiv, Odesa, and other major Ukrainian cities to force a change of government in Ukraine. That campaign has culminated.
The culmination of the initial Russian campaign is creating conditions of stalemate throughout most of Ukraine. Russian forces are digging in around the periphery of Kyiv and elsewhere, attempting to consolidate political control over areas they currently occupy, resupplying and attempting to reinforce units in static positions, and generally beginning to set conditions to hold in approximately their current forward positions for an indefinite time.
Stalemate will likely be very violent and bloody, especially if it protracts. Stalemate is not armistice or ceasefire. It is a condition in war in which each side conducts offensive operations that do not fundamentally alter the situation. Those operations can be very damaging and cause enormous casualties. The World War I battles of the Somme, Verdun, and Passchendaele were all fought in conditions of stalemate and did not break the stalemate.
Ukraine’s defeat of the initial Russian campaign may therefore set conditions for a devastating protraction of the conflict and a dangerous new period testing the resolve of Ukraine and the West. Continued and expanded Western support to Ukraine will be vital to seeing Ukraine through that new period.
The reason I am citing this is that weapons that have served Ukraine well so far, such as anti tank missiles and man portable surface to air missiles will not be as useful to neutralize entrenched artillery deliberately pounding civilian centers from a distance.
Neither will jet aircraft, not over airspace contested by advanced Russian SAM systems, so let's not over-focus on solutions in search of a problem.
p.s. For what it's worth, the debated Mig 29s are also near useless in this context. Russia would just pound Ukrainian airbases from a distance and the Ukrainians do not have enough pilots or the infrastructure to train them in a timely manner. The bulk of Ukraine's current woes are due to Russian ground forces, such as artillery and missile strikes and the Mig29, being an air-to-air superiority fighter isn't useful there. Yes, every bit can help, but it's like buying a Mustang to go off-roading: missing the point.
p.p.s. to steal a point from Zomvid-21, Vietnam was not Ukraine in another sense as well: Vietnam was far remote from the USA, whereas Ukraine is pretty close to Moscow. This would be more analogous to Russia intervening in a conflict in Canada, Mexico or Cuba. So, whatever the level of Russian duplicity and bullying inherent in the "special military operation", NATO needs to be very careful in delivering mostly defensive weaponry, rather than gear that can menace Russian territory.
Vietnam War was a textbook example of a safe proxy war: very remote from both belligerents, over a country that wasn't of any major significance to either of them, and with a very sharply defined scope. It would be reductionist to call Vietnam just a pissing match between two superpowers, but it was a relatively safe (at the expense of someone else's lives) way to test each other out.
Ukraine is in many ways the opposite. It's close not just to Russia, but to Moscow specifically. It's of special significance to one of the powers, to the extent that a government that loses it may not survive. The scope of the war can easily spill out to neighboring countries, such as Belarus or Poland.
The closest historical analogy was the Cuban Missile Crisis. A closer fictional analogy would be a hypothetical war over Canada, if it somehow veered so far to the left as to consider joining the Warsaw Pact. Such a prospect would have threatened the US to an intolerable extent, allowing weapon placement close to DC, and leaving Alaska surrounded (Belarus has had that concern).
The Soviet Union assisted Cuba to a great extent, but not to the point of starting a shooting war; it even asked Cuba to avoid one. They would've been even less likely to do so with Canada. Vietnam had very little escalation potential, because it only threatened influence over a neutral region, not national security.
Some resemblance to the Cuban playbook can be seen in this conflict: consideration of extreme options, an economic blockade of Russia, and some degree of brinksmanship. It also has to deal with indirect control on both sides (LDNR and Kadyrovets forces, Azov Battalion), meaning that events can happen that neither nuclear power desires, as has been the case in Cuba.
Sending jets to Vietnam presented no risk that they'd be used against the US or the USSR. With Ukraine, there is no such assurance, and it has shown unexpected resolve in the war. It is possible that such jets, if they survive, will hit targets outside of Ukrainian borders.
With sufficient escalation, nuclear force may be used in the conflict.
Note that MAD is not the only way to conduct nuclear warfare. Nuclear weapons are exceptionally versatile. They can be detonated on the ground in the old way, at medium altitude to limit fallout, at high altitude to cause urban firestorms, or in space to produce electromagnetic pulses.
As long as the weapons are not carried by ICBM or MRBM, they can be used without alerting national missile defense grids. If sufficient warning is given, such use is unlikely to result in a MAD response by a rational actor. It would violate non-proliferation treaties, but would not under common definitions be considered a nuclear attack against a NATO member.
Update: A March 7, 2022 Congressional Report highlights this specific risk:
The 2018 Nuclear Posture Review affirmed that Russia maintains and is modernizing “an active stockpile of up to 2,000 non-strategic nuclear weapons.”
There is widespread agreement that Russia is pursuing a broad-based modernization program for its nonstrategic nuclear weapons, although experts disagree on the pace, direction, and rationale for this program.
Russia’s doctrine indicates that it would use these weapons in response to a weak performance by its conventional forces in an ongoing conflict.
A weaker than expected performance by conventional forces is already happening. But as long as it's strictly an offensive operation, Putin has reasons to hold back. If fighting spills over to Russia or Belarus, the stakes go up.
Second-to-worst-case scenario, a war of this magnitude may lead to limited employment of tactical nuclear weapons. EMP blasts in particular can take out electronics over a wide area, without directly killing people on the ground.
Such non-MAD operations can lead to further nuclear proliferation and a gradual de-tabooing of nuclear warfare. And this will matter long after the Ukraine affair is over.
Several reasons:
Germany is freaked out about it, and totally opposes it. For Germany just sending AT missiles was a big policy change. Before the actual invasion, they even opposed their allies from sending any weapons that had German components. So this is ostpolitik in another incarnation. One taboo removed, but others remain. Unlike in Vietnam, the US does have to care what its continental allies think about this.
Poland was freaked about it too, but in a different way. It didn't want to be seen as making that decision largely by itself, even after the US gave them a "green light". So they tried to involve the US & the rest of NATO as much as possible (by sending the planes via the US base in Germany). Apparently nobody wanted to be seen as their base being the place from where the planes last took off towards Ukraine.
The Soviet-era planes (which Ukrainian pilots know how to fly already) that available in Poland (or Romania) are fairly old and limited in number. And even then, the Polish version has different avionics and mission computers, having been locally upgraded. It appears that Poland has few long-range radar AA missiles that could be fitted on the MiG-29, so they'd be limited to their short-range IR ones, which would put them at quite a disadvantage against Russian fighters. (There's a factory in Kyiv [Artem] that makes these missiles, but it has been hit by Russian strikes.) So their impact on the battlefield would probably be quite limited.
Training Ukrainians to fly Western jets would take a substantial amount of time. As other answers have elaborated, the North Vietnamese Army had substantially more time to prepare its pilots and stock-up on planes, before and during the war, because the US escalated the war on North Vietnam itself rather gradually. The Haiphong harbor (where the Soviets were unloading most of the supplies) was not mined until 7 years into the war, for instance. Russia is already striking airfields, aircraft parts factories, and supply depots near the Polish border. And they've totally blockaded the Ukrainian coast (although that would not matter for planes).
Long-range precision weapons have gotten better since the Vietnam era, making airfields more vulnerable. The US didn't manage to hit NVA airfields with cruise missiles back then, as far as I remember.
As a consolation prize, the US sent some 100 Switchblade drones. These are short range loitering munitions that could e.g. take out artillery. They need much less on-the-ground infrastructure to operate in Ukraine than fixed-wing aircraft do.
International law has a concept called jus ad bellum, describing when and how a country is permitted to go to war. There is separate from jus in bello, which describes how war may be conducted.
And then there are principles of realpolitik. One of those principles is that wise countries don't back a nuclear power into a corner it can't get out of. Ever. And a loss in Ukraine would be much more serious for the current Russian administration than a loss in Vietnam would have been for the US administration.
Western anaysts believe that Russia would use nuclear weapons to prevent a conventional defeat on Russian soil. Also note that Russia considers Crimea part of Russia, while Ukraine and much of the rest of the world consider it part of Ukraine. This mirrors NATO strategy from a time when NATO had inferior conventional forces and relied on the threat of nuclear weapons to keep the Soviets in check.
On the other hand there is the madman theory of international negotiations. One appears to be willing to kick over the game board rather than accept a loss, and this appearance influences the opponent. How much are you prepared to bet that Putin is just bluffing? And whatever you think, how much are the Western governments prepared to bet?
There's several moving parts here
Vietnam was a proxy war
A proxy war is an armed conflict between two states or non-state actors which act on the instigation or on behalf of other parties that are not directly involved in the hostilities. In order for a conflict to be considered a proxy war, there must be a direct, long-term relationship between external actors and the belligerents involved. The aforementioned relationship usually takes the form of funding, military training, arms, or other forms of material assistance which assist a belligerent party in sustaining its war effort.
There were many such efforts where the Soviets and the US would help arm one side or the other, depending on who backed them. Ukraine is not a proxy war, because Ukraine has not been a long-time ally of NATO or the US.
One does not simply "give warplanes"
The Soviets had the benefit of time to develop a VietCong airforce
Starting in 1964, North Vietnamese fighter pilots and anti-aircraft gunners were being trained in the Soviet Union, with Soviet advisors also being stationed in North Vietnam.
That's the major difference here. Let's say the US wanted to sell Ukraine F-23 fighters. We likely don't have any shrink-wrapped planes, so you would have to manufacture them. Then you need to train the Ukranians to use them. Both take time that Ukraine does not have. If the war stretches on for years, then maybe this would make a difference, but there would be no sort-term benefit, and the Russians might accelerate their attacks if they know US-built warplanes are coming eventually. That was why the Polish MIG-29 fighters were potentially useful: the Ukranians already knew how to use them.
All questions about Vietnam, or WW3 has zero relevance. The reason why America won't do that is more basic than any of that, or how Putin might/mightn't react. The simple fact is that modern warcraft are so specialised that pilots not only have to learn how to fly fighter craft, they have to learn how to fly specific fighters. You can't simply take a pilot out of an Ukrainian fighter, and put them into an American one. They couldn't fly it; it'd be like taking a F1 driver out of the car & putting them on a Moto bike & saying "there you go, compete," only they're going five or ten times faster, are in a full 3d atmosphere, and a mistake means death. And for the Polish fighters, the Poles wanted free upgraded aircraft, and there was still the issue in that Polish fighters are substantially different in its upgrades & specs to the Ukrainian fighters.
Of course there are the smaller things like technology to protect & the like.
You could say that America wants to see Putin bleed on his own without dragging the west into it, but that's not important for the aircraft issue, fighters wouldn't be an option unless Americans were flying them regardless.
