Got your back

A very academic explanation of this phrase is found here.

A military version of this phrase is "got your six." The saying originated with World War I fighter pilots referencing the rear of an airplane as the six o’clock position. If you picture yourself at the center of a clock face, the area directly in front of you is twelve o’clock. Six o’clock is what lies behind you. On a battlefield, your “six” is the most vulnerable. So, when someone tells you that they’ve “got your six,” it means they’re watching your back. By extension, that person expects you to have their back as well. “Got your six” is now a ubiquitous term in the military that also highlights the way military members look out for each other. (source)