In WWI, the was slow and unreliable, but effective.

Tanks.

Explanation:

During the world war I, the development of mass weapons of destructions still haven't reach the current state. So, the armies at that time still relied on automatic rifles/guns.

Tanks provide a very strong protection for this type weapons'

It provided a way for an army to pernitrate enemy's territory which filled with automatics turrets/riffles as their main source of protection. Compared to air planes, tanks were significantly slower. But it's still very effective to opened up a pathway to bring down a milltuiary base.