simple past tense and past participle of ally
"The two nations allied against their common enemy before signing the peace treaty."
In plain English: To ally with someone means to join forces and work together toward a common goal.
"The two countries allied against their common enemy during the war."
of or relating to or denoting the Allies in World War II
"an Allied victory"
"the Allied armies"
of or relating to or denoting the Allies in World War I
"an allied offensive"
"the Allied powers"
united in a confederacy or league
"The allied tribes formed a confederacy to defend their shared borders against invaders."
joined by treaty or agreement
"The two nations became allied after signing a mutual defense treaty."
Joined as allies.
"The allied forces advanced across the trenches together during the final year of the war."
relating to the Allies (allied nations during World War One)
In plain English: Allied means working together with someone else to achieve a common goal.
"The two countries formed an alliance to fight together against their common enemy."