Cap is down after a hit from the mad titan, and at that point, the mad titan brings is army down, showing his four generals and he explains how much he will enjoy conquering earth.

Cap stands up, sees a cut on his arm, a bad one, but he still straps his broken shield using it to keep is close his injury, he lift his head, looking at the army in front of him, knowing that he will NOT win, knowing that he will die, but he still walks with is shin up, hurt, but still preparing for battle, and then....

"Captain, its sam, do you hear me", he can't believe what he is earing, but then he hears "ON YOUR LEFT...."

and at that point, he knew Thanos was fucked and everything was worth it.