Explaining the joke a bit: if there is a surprise attack, the military only requires you to be wearing three things before you go help repel it: your boots or some footwear, your ammunition or equipment belt, and some body clothing. These days, they also require you to be wearing at least your armor vest and helmet as well.

This soldier has put on her boots, her belt, and a long shirt and is firing back at the enemy, but the hurried preparation means she's not wearing anything down below, and her stance is giving the guy next to her a wonderful view of her privates.