The US killed Soleimani without notifying Congress, which a lot of people in Congress are angry about. There are fears this could start a war, which would constitutionally require congressional approval to be declared. However, if they wanted to kill Soleimani, is there a way they could have approved it and a potential war privately, in a way Iran wouldn’t hear about in advance?
Is there a legal way that Congress could vote to approve a war secretly, so that the US could strike first?
Yes, hypothetically...
Congress could convene in a closed session. During a closed session, everything that Congress discusses and votes upon is supposed to be kept secret.
There have been quite a few of these during American history: https://en.m.wikipedia.org/wiki/Closed_session_of_the_United_States_Congress
...but this likely wouldn't be all that effective in maintaining surprise...
People might not know what specifically the results of a closed session would be, but it is usually publicly known why a closed session is held in the first place.
Plus, any given military intelligence apparatus would know if the US was planning to go to war with Iran just by paying attention to what US troops are doing. There would need to be significantly more forces in position to attack Iran than there are now, if the point would be to actually invade the country.
... and it would be domestic political suicide for Congressmen.
"Should we go to war or not" is the biggest issues any given country can face, doubly so for representative democracies. People who are elected to Congress often make promises about how to vote on foreign policy issues; it would be a really, really bad look to tell your constituents "I would prevent us from getting into any more foreign wars" and then promptly turn around and hold secret votes on that topic.
No. Congressional votes are not inscrutable to the public and one can find any member of congress's entire voting record.
That said, appropriations can from time to time produce "Billion Dollar Hammer" for want of a better term. This occurs when Congress appropriates money for a program that is given a cover term that doesn't describe anything and the only items that are visible in the budget are typical simple tools (i.e. the Hammers or a classified sites' toilets) when they appear in public. Typically these go to an agency (the DOD has a lot of these as the funding usually goes to classified projects or R&D for new weapons tech). However, which department gets what money is still outlined.
It is a War Crime to launch a "surprise" war, and war must formally be declared before any war time engagements can be made. Legally speaking, the U.S. Congress has only declared war five times in the nation's history (The War of 1812, the Mexican-American War, The Spanish-American War, World War I, and World War II)... everything else is authorization of use of military force (AUMF). Typically, these AUMFs will be specific to nations or regions, but the AUMF being used to justify the drone strike is the the AUMF against terrorists, which is pretty broad and allows the President to use military force against "nations, organizations, or persons he determines planned, authorized, committed, or aided the terrorist attacks that occurred on September 11, 2001, or harbored such organizations or persons, in order to prevent any future acts of international terrorism against the United States by such nations, organizations or persons." In addition the AUMF Against Iraq also gives more targeted focus to the nation of Iraq.
Suffice to say "war" was declared.
Additionally, both AUMFs must comply with the War Powers Act requires the President to notify Congress with in 48 hours following the attacks, which he has, and all operations must conclude within 60 days of the initial attack unless an AUMF or declaration of War are passed by Congress. As one person described it to me yesterday, Congress pays for the Sword, while the President swings the Sword. Thus, Congress can always pass a law pulling funding from operations they do not like or sue in the Supreme Court for not following the law.
And keep in mind, the Surprise War Declaration is very important. It's the reason why Pearl Harbor was "a date which will live in Infamy." It was wrong for the Japanese to start the war, but it was more wrong for them to prosecute the war before the Japanese Ambassador to the United States was actually able to decode and give the Japanese Declaration of War on the United States. Japanese leadership was prosecuted for this following the conclusion of World War II and found guilty. What's more, Britain loved this because they could claim the war crime despite the simultaneous attack on British Colonial assets in East Asia and the Pacific. Technically, Japan declared war on Britain on the 8th of December because those attacks lied west of the International Dateline. Because the Declaration was dated to the 7th of December at GMT, even if they were late, they were still "Declaring War before attacking"... it's a loophole but it was one that could be exploited. However, Hawai'i lies East of the Date Line and since the formal Declaration wasn't given to the Secretary of State until well after Pearl Harbor attack was over, the UK could claim that as a War Crime, as they had a Defensive Pact with the United States, which essentially is a fancy way of saying "We consider any act of war against the United States to be an act of War against the United Kingdom" and since Japan didn't declare on either before Pearl Harbor, it technically meant the War on the U.K. was undeclared before the U.K. was engaged).
TL;DR: Declarations of War might ignore time zones of the capital of the defending nation, but the Hostile Nation cannot attack you before they let you know about it... and Japan screwed up in WWII.
The reasons for a closed Senate session don't seem to include declarations of war, or the budget it would need.
You would likely need the support of a majority of Senators and House members to do this. Each part of Congress can set its own rules, and you would need a rule making the process closed-door. Of course, you could not keep the press from reporting on it, or asking questions of members, and the fact that war was declared would also be hard to keep secret.
I suspect you might be asking a wrong question. Here's why:
It's possible that a nation (say, Foonia) could come to a decision, internally, that a state of war existed between Foonia and its ancient enemy Barbia. However, the term "declaration of war" implies that this information is being shared with ("declared" to) Barbia. And this makes sense from the perspective of a well-functioning international politics: if Foonia can be at war with Barbia and not tell them about it, then all nations are at effectively war with all nations at all times. This is not a healthy state, and not one that's conducive to the sort of peaceful coexistence that is a pre-requisite for gettin' stuff done, which is presumably the goal of most nations most of the time.
Now, it's obvious that the current state of international affairs is one where nations feel relatively free to conduct warlike acts against each other with little or no warning (US attacking Iran without provocation, Russia seizing the Crimea) and where there are unhelpful ambiguities of territory (Kashmir). So the notion that a declaration of war is required is, realpolitik-wise, a bit naive and idealistic. Obviously, the US can attack other nations with relative impunity, and get away with it. But this does not mean that this is a long-term tenable state of affairs.
Ultimately, the whole point of "declaring war" is to ensure that nations do not commit surprise attacks against other nations. While it might be possible to come up with a legalistic framework for the US to satisfy itself that an assassination of a foreign national is a "legal act", such a justification could not satisfy any framework of international law worth having and would make the US an international pariah state.
The US can strike first any time it wants, we do it all the time. Now, that is technically against international law (and note, I am not taking a position on either side of this, just making a statement). But a Declaration of War, by the very definition of the word "Declare" can not be secret. So no. We can strike first, but we can't do so based on a secret "declaration" of war, and argue what we did was within the bounds of international law.
Also, in response to Commenter Adam Miller, who asked "Why do you say that Congress is "required by the constitution to approve" a war?" I suspect that it is because that is what the constitution says in Article 1, Section 8, Clause 11. https://en.wikipedia.org/wiki/War_Powers_Clause
Of course, Congress doesn't "approve" a war, so the question is worded slightly incorrectly, so perhaps that was the question.
Also, Commentor Justin Lardinos I think is confusing two concepts. One is a declaration of war, which congress has the sole power to make, and the other is the presidents duty to notify the Gang of Eight in regards to certain military action.
This conversation can quickly digress into discussions about AUMFs, and is "declaring war" even a thing anymore (our last declaration of war was against Hungary, Romania, and Bulgaria in 1942. Yet we engaged in conflicts anyone would describe as wars in Korea, Vietnam, Iraq (twice) and Afghanistan since.
This happened. Right off-hand, I can think of twice.
First, the USA sided with Britain against Germany before the formal entry into World War 2. Through the use of the Lend-Lease program, the USA was supplying Britain, effectively contributing to just one side of the war effort.
Second, there is Charlie Wilson's War. Charlie Wilson was a congressman who obtained funding, through votes in congress, to covertly finance military action in Afghanistan against Soviet forces. Significant amounts of funding were vague or mis-described, but the ultimate point was to manage to fund military action. The voting congress people knew full well just what was happening, but the official records were not clearly describing what was really going on.
Through these votes, the USA was effectively participating in the war, despite no formal declaration of war. And, in the second case I mentioned, this was done covertly, so the enemy might not know just what exactly was happening.
Considering these events, when I look at the two questions asked, I reach the conclusion that the answer is "yes".
Is there a way that Congress could secretly vote on approving a potential war?
Is there a legal way that Congress could vote to approve a war secretly,
In neither of these cases was there a formal declaration of war before activities occurred. There might be no way to secretly make a formal "declaration". (To "declare" seems to be the opposite of hiding the information.) However, since approved actions were causing active military action to be able to occur, I would consider that to be approval of war activity. (Certainly, the first question's looser text of "approving a potential war" is satisfactorily met.)
For aggressive actions against another state, the state needs an amenable bureaucracy which can execute the wishes of the state's leadership if the leadership currently doesn't have the parliaments expressed approval.
I don't believe that Congress had approved the plotting by ambassador Henry Wilson in 1913 against president Madero in Mexico which led to the executions of Madero and his brother.
The same with the other deployments of the Marine corps in Guatemala and Venezuela, war-like actions YES.
Prussia in the 1860's is another example. The parliament declined Bismarck's proposals to raise taxes so that the army could be reorganized and enlarged. The king and chancellor Bismarck's answer was to order the bureaucracy to ignore parliament's decision, continue collect the higher taxes and transfer the funds to the army. The army staff recruited more soldiers and got them their supplies. In 1863 due to the disagreement between the King and Parliament regarding the budget, the King dissolved the parliament under the pretext that Parliament had intruded on the ministry (which according to the constitution was his responsibility.)
In 1864, due to the death of the current king of Denmark who also was duke of Holstein-Schleswig (which was a part of the German confederacy) and disagreements in Schleswig and Holstein about their belonging as a danish ducal fief or part of a future enlarged Germany (Prussia/Bayern) Prussia and Austria declared war, invaded and was able to defeat the danish army (which was as large as the standing Prussian army but which was enlarged 60 percent with auxiliaries.)
In 1865 Prussia forced Austria to war due to a dispute regarding the administration of Schleswig and Holstein. Austria lost and was forced to leave the German confederacy. In the election in 1866, the war successes and unification meant that Bismarck was able to discredit the previous liberal majority of Parliament. The election resulted in a parliament with a large conservative nationalistic majority. The new parliament acceded to the previous 4 years of over-taxation.
