Initially used to gather intelligence about the enemy there's a machine gun, tank, trench warfare, u-boat, battleship chemical weapon, gas mask airplane

Explanation:

If you are gathering intelligence, then you are passive. You don't want to be noticed. That lets out machine guns, tanks, trench warfare, battleships gas mask (which is purely defensive)

The two that are left out are airplanes and uboats. Towards the middle of WWI airplanes became offensive weapons.

I'd pick uboats but it depends what time period you are studying. These were not offensive weapons until WWI. They were used to gather intelligence about the enemy during the civil war.

The problem is that planes, before 1915, were used to gather information.

I'll stick with uboats, but recognize this is kind of iffy.