The US had to declare war without Congress, because Congress basically agreed to create a unified Korea under a democratically elected government... an election that the communists won and caused the US military to panic.

As the US had just finished WW2 and Cold War propaganda had not really taken effect yet, the population was vehemently against any other war, especially as the Soviets and Chinese were still seen as good friends and allies. A problem the government... circumvented.

You should check out the podcast 'blowback', their last season was all about the Korean war. Over ten hours of great, us-critical (i.e. fair) history and unlike their other two seasons I was shocked by how little of this stuff I remembered/was taught in school

Now the US has just learned to bomb countries with drones instead of bothering to declare war, save themselves a lot of time. That way they never need to bother with inconveniences like approval or popular opinion.