This week at Fort Benning, Ga., the Army told some 200 industry representatives from 59 companies what it wants in its next war machine, the Mobile Protected Firepower vehicle (MPF). The MPF must be light and nimble enough to accompany foot troops where the massive M1 Abrams cannot go: into dense jungle and narrow streets, up mountains and over rickety Third World bridges. It must also carry enough armor to shrug off .50 caliber machinegun fire and some heavier weapons. It must pack enough firepower to take out anything from a concrete bunker to a heavy tank. That’s ambitious. Combining mobility, firepower, and protection in a lightweight and affordable package is hard — so hard that this is the Army’s fourth try in 20 years. The M8 Armored Gun System was cancelled in 1996 to save money. The Future Combat System was cancelled in 2009 because its ambitious high technology wasn’t working. The Stryker Mobile Gun System (MGS) managed to enter service, but to widespread dissatisfaction. The Army is determined to do better this time.