Any of those instances where you pause for a moment and contemplate the purpose of war as a young man in his prime is gunned down by enemy fire in front of you.
The ending movie of the first Call of Duty.
Vic's death in Big Red One. If you've played this game, you'll know why. As well as Hawk getting medevaced and Brooklyn's death.
Call of Duty 3
The penultimate level for the 1st Polish Tank Division in Call of Duty 3 ("The Mace"). Facing relentless German attacks, your tank is soon disabled and you're forced to fight on foot. Then, your companions are killed one by one as the defenses collapse one after another. Finally, just when all hope seems lost, green flares pop in the sky and Canadian reinforcements arrive. What made it so tear-jerking, however, was the knowledge that all this was faithful to real history. The Poles really did give their all in that battle.
The death of Isabelle at the end of the British campaign is enough to cause The Stoic Marcel to break up completely. Keith, who has less-than-flatteringviews on the French and didn't even try to hide it throughout the campaign, comforts him with "She was a brave lass."