What has China gained by doing military drills after Nancy Pelosi visited Taiwan? A lot of people who know nothing about geopolitics are saying that China is throwing a tantrum, but is this really the case, or is it just some geopolitical move where China gains something by doing this, and the U.S. loses face and is forced into a corner?
What did China gain by doing this, what did China lose, what did the U.S. lose and what did the U.S. win during the visit and the following military exercise?
China's military exercises around Taiwan aren't mere posturing. Xi has vowed to retake what he regards as a breakaway province. China is practicing for the upcoming military operation to secure its view of "One China".
What China gains by conducting these maneuvers is asserting them as acceptable. They were extremely close to Formosa's shores. These actions amounted to a partially-enacted blockade, with actual disruption to shipping.
An invasion would not be the most effective way to conquer Taiwan. Being an island, with very limited natural resources, it's dependent on external trade for its food, energy, fuel, and raw materials to produce anything it might be able to trade for these.
Only 6% of total and 16% of electric power in Taiwan comes from import-independent sources. It has three nuclear reactors and a minor amount of renewables, which isn't enough for the essential services. Taiwan is a net food importer, with the majority of their imports coming from China, followed by the US.
China's conquest of Taiwan would most likely be through a blockade. However much people might value freedom, they value food and roof over their head more - modern cities become uninhabitable without power. China knows this and is likely to use it, be it Xi or someone that comes later. They're drilling for just this.
P.S. Under international law, blockade is a war operation. It's distinct from an embargo, which is the strongest non-war measure against trade.
Once a blockade has been proclaimed, "a public notification given by the belligerent to a neutral government is ordinarily sufficient to convict all subjects of that government of the requisite guilty intent", should they attempt to run the blockade.
To continue citing the law, "The vessels of neutrals are affected, and this right of a belligerent is universally admitted." The cargo of a ship guilty of breach of blockade is subject to confiscation, as well as the ship, with a few exemptions.
Actions taken for the enforcement of a blockade remain part of the war between the blockader and the owner of the blockaded port. Ships attempting to run it would be subject to attack or seizure without creating a state of war with their owner or flag nations.
It's been a while since these laws were invoked, of course. Today, wars are rarely even declared. What this mean in practice is that if neither China nor US want to go to war with one another, they don't have to, even with one attempting to run the blockade and the other enforcing it.
A lot of international politics — not all, but more than one might like — plays out as middle-school level power/status games. More precisely, it comes down to interactions between smart, cagy, ego-driven, and effectively amoral participants. It's incorrect to describe behavior on this level as 'tantrums' because the term 'tantrum' implies an uncontrolled emotional response. If an age-typical middle-schooler shows something that looks like a tantrum, we can rest assured that it is behavior calculated to appear that way in order to produce a certain type of social pressure.
In this particular case, China has been steadfastly and peremptorily claiming Taiwan as part of its territory. This is a standard bully-tactic: claiming something as one's one, and then using brusque menace to keep other people away from it. The idea is that (eventually) other people will give in to the menace and allow the bully to have what it wants by default. In other words, China has been trying to establish de facto dominance that will eventually become de jure dominance as other nations shy away from conflict. When Pelosi went to Taiwan, she effectively challenged that claim; she knocked the chip off the bully's shoulder, as the saying goes. This put China in a bind. They could not allow the challenge to go unanswered, otherwise the entire strategy for bullying Taiwan into submission falls to pieces. They also could not attack Pelosi or the US directly because that would have risked a direct military response from the US, which is more than China wants at the moment. So they did the only thing they could do: scale up their aspect of menace so that everyone knows they are still claiming Taiwan as their own. And so we get massive military exercises, angry recriminations against the US, and other snippy, chest-beating behaviors.
There's nothing here more sophisticated than (say) a jealous boyfriend puffing his chest out and flexing his biceps because his girl talked to another guy, and the message is the same: "stay away from what I claim as mine, or you're gonna get it".
At the very least, the maneuvers are designed to mollify the ultrantionalists who perhaps expected (after a tweet by a well-known Chinese editorialist suggesting such a move was "ok") for Pelosi's plane to be shot down. (His tweet has since been deleted. The longer editorial he wrote in the actual newspaper only suggested that PLA planes enter Taiwan's airspace to escort Pelosi's plane, all the way to the landing.) In any case, the PLA itself had promised to "not sit idly by" if the visit were to happen.
Besides that, the maneuvers are perhaps intended to scare Taiwan from inviting/accepting such a visit in the future. (Added:) HK-51's answer seems to be correct in that at least some PRC sources have sought to portray the maneuvers as a blueprint for a future embargo:
Meng Xiangqing, a professor at the PLA National Defense University, said the six areas were chosen to show how China could cut off Taiwan's ports, attack its most important military installations, and sever access for foreign forces that may come to Taiwan's aid. "Connect the six areas in a line, like a noose, with the knot of the noose right in the southwest direction," Meng said in an interview with state-run broadcaster CCTV. [...]
Meng noted the northern exercise areas had successfully sealed off Taiwan from Okinawa, the island where both Japan and the United States base substantial military assets. In the southern areas, the PLA showed it could control the Bashi Channel, "the only way to enter and exit the South China Sea," he said. And in the eastern areas, China's forces showed that accurate Chinese missile fire could force foreign warships to back away from Taiwan's waters, he added.
"This is an unprecedented encirclement of Taiwan Island," Meng said.
So yeah they are claiming to have showcased "unprecedented" capabilities.
China may have used the maneuvers to exercise attacking Taiwan and gain some more first hand experience there. But they also showed that short of invading Taiwan there is probably nothing they can do to convince Taiwanians that they are the good guys here. It shows their lack of other options.
The US has shown that they stand by Taiwan (to some extent) but have China given a pretense to flex their muscles. Next time, when China may really want to invade Taiwan, the US should be better prepared and be cautious to not give China too many such cheap excuses.
Military Operations always entail a degree of risk. A chance of messing it up, botching the execution, or otherwise causing the operation to fail.
The more risk that there is in a given operation, the less credible that operation is. For example, if one imagines a hypothetical invasion of the Continental US led by, say, the armed forces of Italy, it does not seem likely to succeed. Therefore, the US is acting reasonably when they discount a threat of invasion from Italy, based on current information.
To get back to China and Taiwan; China does not consider Taiwan to be independent, and have repeatedly asserted that they do have a right to secure the territory, by force if necessary. The US (and others) are required to evaluate this threat's credibility. If the US believes that the threat is credible, then they will be forced to assign a greater weight towards actions based on diplomacy and compromise. If they feel the threat is not credible, then the US is free to set an aggressive or confrontational foreign policy.
For China, then, to execute military exercises simulating a blockade or invasion of Taiwan, is a statement of credibility. They are saying, in effect, "Look here. Pay attention. See how we have just run a drill, a realistic drill, wherein we have practiced a successful invasion of Taiwan? We have done so openly and in full view of your spies and satellites. You know, just as well as we do, that this drill went well. Therefore, you now must accept that we have the ability to launch a successful invasion of Taiwan."
Their objective is to force US planners and policy makers to realize that the invasion of Taiwan is something that China has within its likely capabilities. In the absence of these military maneuvers, policy makers may, in effect, say, "I know China wants to invade Taiwan, but realistically, they can't, so we're free to ignore their demands.". Now, having seen a successful military drill of an invasion of Taiwan, these same policy makers are forced to say, "I know China wants to invade Taiwan... and it turns out that's something they could realistically do... so rather than risk it, we should probably be more diplomatic and considerate of their position."
So the military operation achieves a real-world result that is beneficial to China's interests, which is why they would undertake such a costly and risky operation. Its why we frequently see these in times of tension, from all kinds of international actors. They need their rivals to appreciate (and hopefully be deterred by) their ability to project force.
