Borrow the Road to Conquer Guo
Borrow the resources of an ally to attack a common enemy. Once the enemy is defeated, use those resources to turn on the ally that lent you them in the first place.
Be wary of those you hire contract to permanent. There was a colleague of mine who came to my company in this manner. He was as nice as he could be. He was endlessly helpful; since he made my life so much easier I rewarded him by helping him navigate the company politically. Because of my help, he was hired permanently, and that's when he immediately showed his true colors. He started to badmouth me behind my back to his boss, for whom I worked in a matrix situation. Of course I was able to parry his attacks, but it wasted my valuable time and energy. Of course, it served me right. I should have given him nothing in the first place; I was too generous, and that always comes back to bite you later on.