The Constitution says it has to be a "Declaration of War" from Congress to go to "war",
Some have, I believe, argued that, back in 2003 when the vast majority of everyone in Congress voted for the Iraq war, THAT was indeed, sufficient to meet the Constitutional requirement for Declaration of War.
The Constitution provides no detailed instructions with regard to the format of the Dec of War. Does it need to be in writing? Oral vote? Const. does not say.
So, if someone were to argue to you that, in 2003 Bush had BOTH UN Security Council Resolution support AND Congressional Declaration of War, the correct response is: "Doesn't matter, it was still a dumb idea to invade Iraq, even if it was not blatantly illegal under US and International law."