1. To assume a position of power over a person, group, or thing. The terms of the treaty keep our country under the king's control. I think you'll have a hard time keeping the department under your control—they're very loyal to their old boss.

2. To have power over someone or something so as to limit their or its potential negative effects. In this usage, a pronoun is not used between "under" and "control." Thankfully, the teacher was able to keep all the kids under control during the evacuation.