Theoretically congress can sort stop something from happening. But the president can deploy the Marines w/o anybody's consent as well as launch nuclear warheads. But right now republicans and the tea party control both houses of congress so there is no opposition with any power at all. He is absolutely the most powerful person on the planet. He also flies into rage and calls for violence against anybody who criticizes him. Do you honestly think that al-Qaeda and ISIS won't want to push his buttons so he'll do something crazy and make the rest of the world hate us? And when that happens he'll just continue to double down until we have WW3. Putin will work him like a puppet and things are going to get very bad.
When W was elected I was right when I guaranteed we'd be invading Iraq and that was nothing my friend. When one of our boats drifted into Iranian waters both Trump and likely Secretary of Defense Ted Cruz called for us to unleash our full military on them. Meanwhile John Kerry negotiated their release withing 24 hrs. There will be no more "Let's talk to them and see if we can work it out", we'll be invading other countries for the slightest sense of a threat.