The US government already has the authority, or "international permission" to go to war with China. The international court in the Hague already ruled against China regarding the aggressive taking of the Paracel islands and the Sprately islands away from several nations that have legal claims on many of those islands and fishing grounds. The ruling was established and recorded, but China refused to abide by it. That means the US can act on those rulings at anytime and have an international legal body giving them the authority to take actions to "correct the crime" that has already been made.
With the recent sinking of a fishing vessel owned by Vietnam, the US has a strong justification to take offensive actions to correct the injustice that was done by China. With one action, like the small invasion of one of those islands to expel the Chinese invaders, the beginning of a conflict would ensue.
The US can claim that the Chinese Navy took "pirate" type actions against the legal owner of an island and the fishing region in the ocean that legally belonged to the nearby nation, in this case being Vietnam. Vietnam has already incurred a loss, one of many through out many years. In some respects, any action that the US would take could be correctly identified as a "defensive action" by the US, and give them, in their mind, validation and official sanctioning of their actions. Because of this on-going conflict in the south China sea it requires us to consider this a major hot spot for further conflicts, which could lead to a major confrontation between the US and China, and possibly with the allies of communist China as well. George Eaton

https://thediplomat.com/2020/04/south-china-sea-us-state-department-criticizes-china-for-reported-ramming-sinking-of-vietnamese-fishing-boat/