Well, congress is the only establishment which can declare war, not the president. The president can call a meeting and try and get congress to do so, but the president cannot issue a declaration of war. The problem with this, though, is that there are no checks and balances, like there were set up to be; all three branches of the government work as one, instead of balancing each other out. The only reason that Trump, Obama, Bush, etc. had so much power is because we the people let them, along with congress.
But, WWIII is imminent, I believe. Between tensions in the Asian countries, between Europeans and Arab immigrants, or between Russia and well...everyone...the fall of economic systems in Europe and in the US will inevitably lead to conflict, whether it be revolutions or world war.