As verb: To give your support to another group or country (alliance, league, align, affiliate)
As noun: A country that has agreed to help and support another country, especially in case of war(or a person who helps and supports sb who is in a difficult situation, especially a politician)
Tom found an ally in Bill, who backed him on all issues.
Russia allied itself with the United States during World War 2
The prince allied himself with the Scots
a close ally and friend of the prime minister
his sister was his ally against their grandparents
our European allies