This idea is used in Empire of the Sun by Andrew Weiner, which as far as I can see has only been published in Harlan Ellison's anthology Again, Dangerous Visions and Weiner's own anthology Distant Signals and Other Stories. The story starts:
Earth, year 1990, falls victim to the history-bending project of the Sirius Syndicate. Travelers in time establish a network of communication satellites around the globe, and open the Martian War. Continuous 'Martian' broadcasters promise destruction, total war, liquidation, war without limits, to the terrified Earth populace. TV channels are jammed with further images of death. The United Nations is revived, as the World Draft Board. Military elements seize control, promising "the War of all Wars." A space armada is to be constructed. The first troop draft is announced; twenty millions. Draft rioting in China is vigorously suppressed
The relevant scene is:
Kaheris weaves his way through the dead and up to the summit of the hill, adrenalin driving him on, blind to his terror. The hill must be taken. Sure. One of the enemy sprang from a dug-out. Tossing aside an empty gun, it came at him with a knife. Physically, it looked human.
It screamed "Dirty Mart" as Kaheris shot it, through the head. A trick? He was beyond thinking. Nausea welled up. He sank to his knees.
My only reservation is that Empire of the Sun is far from the usual simple SF story. It's highly stylised (like many of the Dangerous Visions stories) and meant as a condemnation of all war. If this is the story I can't help feeling you would remember how unusual it was.