He can't declare himself dictator as that would be a violation of the constitution, Particularly the rule that says you can't suspend habeus corpus(with the exception of invasion), thus subjecting the President to removal from office, one way or the other. I also still contend he would lose with Congress holding the most powers in the fact the US Congress is given the right to declare war(which gives them the right to order a stop of any ISIS bombings in theory), Congress can remove a President, and there is a ruling that says orders given to the military by their superiors can be ignored if they violate the constitution, US v. Keenan where they claimed to only be following orders when murdering civilians and the court found that was not lawful and should not have been carried out, thus upholding a murder conviction against the soldier.

Now we come to the fun of Nuclear weapons, while the President has authority to order a nuclear strike unilaterally, he can't do it alone, it's actually bilateral because the Secretary of Defense must also agree, should he say no, the president can then fire him and ask his replacement to agree, so on and so forth until someone agrees with the President. While that is going on the Vice President, cabinet and congress also have the ability to invoke the constitutions unfit clause and remove the President from office.

He may be everything you fear, but I doubt it though. As to civil war, that's probable no matter what given the polarization of the US population now, we are sitting at roughly 40/40/20(Left, Right, Other) in terms of politics.

I also know since the Korean war they've used a work around to avoid congress and war powers by claiming as Head of the Military the President can move troops at will for defensive purposes, but I figure with a Trump the supreme court will finally rule in favor of the constitution and not the Presidential office that only congress can start the conflict/action not the Head of the Military/President. Yet, there is also the War Powers Resolution 1973 requires within 60 days for congress to approve any actions. I will concede the WPR basically says congress can vote to stop a war but the President can veto that bill, thus continuing their war, so long as they have 34 Senators willing to block an override of the veto.