Engineering Squad
Builders and mechanics in uniform. They carry tools, not rifles.
Engineers stationed at a base speed up its construction, upgrades and repairs. Send a few to a base that is busy building, even an ally's.
In the field they fix the army's damaged vehicles and repair broken bridges and ports. They can also blow up a bridge in the enemy's way, and get their group across rivers quickly.
They cannot fight or capture bases. Keep them with troops who can protect them: if their escort falls, they are lost too.