On the legality front

Congress practically matters when significant mobilization, boots on the ground, money, with high likelihood of many lives lost. Iraq. Not random one-off adventures.

Otherwise modern Presidents have done this thing for decades.

I think it’s more an effective argument to question this as a policy. As in “is there a plan for what comes next”. Congress should be holding hearings and performing oversight to understand whether theres actually a plan and to allow debate.