Remember in the first Captain America movie when his first role in the military was on a tour raising money for the war? That was actually a thing at that point in time. You had to actually convince people to invest or give money to fund a war. The USD was tied to gold in the real world and they couldn't just create more.
Nowadays, we don't have that. A President might have to ask Congress for funding, possibly. And even then, that money might simply be borrowed (adding systemic risk) or printed (devaluing the rest of our money). As a result, the amount of public support required for waging a war (or any huge expenditure) is much lower.
That's a good thing if the people in power are correct despite popular opinion. That's not always the case, though.