Isn't the U.S. always involved in some kind of war? Will this be a legal war? According to the U.S. constitution, war has to be declared by congress. However, that hasn't happened since WWII. Therefore, every war or conflict the U.S. has been involved in since that time has been illegal!
Let's be real. The United States war machine is always clamoring for a war. When aren't they? There is a lot of money to be made by these wars, at least for a few. And no, they don't have to be legal, obviously.
Sources:
https://www.senate.gov/pagelayout/history/h_multi_sections_and_teasers/WarDeclarationsbyCongress.htm
http://www.foxnews.com/us/2017/12/23/theres-war-coming-marine-corps-general-warns-us-troops.html