To strong-arm someone is to force them to do something. When you strong-arm your sister into getting out of your bedroom, you physically push her out the door.
Bullies strong-arm smaller kids into handing over their lunch money, and powerful countries strong-arm weaker ones into cooperating with treaties and alliances. In the second example, a figurative kind of strong-arming is going on; there is no physical action, just the implied threat of negative consequences. You can also use this word as an adjective: "Your strong-arm tactics don't scare me!"