When two things are so closely linked or attached that they act as if they were a single unit, they are united. Allied countries join together during war, united against a common enemy.
If several friends are united against a playground bully, they act as one to fight back or scare the bully away. The phrase "united front" shows up a lot in politics, and it usually refers to groups of people or governments banding together to gain strength or resist some negative force. The United States is a single country formed from many individual states that are joined into one. The Latin root, unus, means "one."