Spells and Shells
When the rift opened nobody knew what it was, until millions of troops came through. when troop finally stopped an estimated 500 million troops were sitting in the Arizona desert. Hasty defenses were built by the US army to stop the sword and shield invaders. It was a bloody conflict, as the army underestimated their enemy. The invaders had what only could be called magic, allowing them to shrug off bullets like nothing. The US army managed a slim victory, hastily charging for the rift. Just as they got near, it closed cutting off wherever it led. Now, fifty years later new advances brought AI into the scene, piloting jets and tanks. New advances in the field of portal tech has given the US army the ability to bring the fight to their otherworldly invaders.