Boots Down

One mechanized combat rig falls. Others respond.
When a platoon of tall boys learns their sergeant's brother is MIA, they go into a city wracked with centuries of war to find him.
By the end of the fight, they will learn of a massive government coverup by the highest officers in the expeditionary fleet, an alien invasion, new portal technology, and an AI uprising that may or may not be benevolent.
All they must do to put things right is save humanity.