And this can be applied in any given situation, a being from the future fighting in a war in the past, will be technologically superior.
If master chief would attempt to fight the war against the Third German Empire alone, he would at first kick ass. But with time his energy shield will fade, his energy supplies will deplete, his ammo supply will run out and his armor will wear out, and then he will die.
Now fighting along side the Americans almost the same thing, except he will have to use eventually US Military issued gear. The Americans would probably not have the means to reproduce ammo or gather energy for his suit. Unless master chief shows how its done and unless they have the technology to gather the raw materials for it in the first place.
He will eventually wear out and become a regular soldier. With the gear and loads of combat experience.
But these stuff are always over the edge questions.
One could ask, what if the German Army in WW2 Invaded America in 1788. Tons to debate and tons of possible out comes.