It seems all pretty normal one day, just your average Army Ranger taking a long walk in the street, shooting Russians and running into a Fancy restaurant for a good time with friends. Suddenly a large metallic thing crashes into the ground not so far away and a few seconds afterwards the Arbiter, Thel arrives. Turns out he was minding is own business as he was about to meet up with the UNSC over some random issue before he entered a slipspace rupture sending him to the past and he didn't even know until his ship accidental malfunctioned. The stunned Rangers still being shot at, not questioning why a reptilian alien from the future is there, thus fight back at the Russians.
Thel starts with a Plasma Rifle (2) a Energy sword, plasma grenades (4) and a Covenant Carbine. How does the missions for the USAR go from there? Also for some strange reason the men higher up trust Thel after he tells one of what he knows of the events, the President also says that they must have him help because (I don't want a alien invasion on my hands here.) he stops after the last US involved mission (Down the Rabbit Hole, MW3) where he will not die but be taken back to the future.
The Americans are allowed to try and reverse engineer his weapons to win the war with the Russians.
Bonus - What would happen if this lasted to 2022 and caused the events of Black Ops 2 (somewhat) Bonus 2 - What if Thel, Rtas, Ntho and Usze are placed in Black Ops Zombies mode and when they all die they go to a different zombies map until they die in all of them.