The War Powers Resolution was intended to back up the Constitution and the writings of the Founding Fathers, which are unanimous on Congress being the branch of government that makes decisions about going to war.
Unfortunately, both the law and the Constitution have been routinely disregarded in recent decades, under the novel theory that the president, as commander-in-chief, has free rein to do whatever he likes with the military in the name of the United States. This is deeply unfortunate, and it isn’t what the law says.
The Constitution, of course, vests the power to decided to go to war in Congress alone. And this is what the law has to say about the decision to go to war: