Allied means united or joined together. If two countries are allied, they are on the same side and have common interests.
When two people are allied, they are friendly — or at least cooperative. To be allied means to have an agreement to work together, so when politicians are allied on a bill, they've pledged to join forces and unite for a common cause. In a war, allied countries are fighting on the same side. Allied comes from ally, which first meant "join in marriage," from the Latin root alligare, "bind to."