In the end, the arms of another will fall from your hand, will weigh you down or restrain you.

What does this mean?

In "The Prince," Machiavelli makes reference to making use of auxiliary armies, either allied or mercenary forces. He suggests that demanding extra forces to provide support to an own army is always risky. The reason is, in victory, the auxiliary army is owed a favor or might act against the party that hires it. Likewise, if the battle is lost, then the employer loses as well.