An alliance is when a group of countries work together in some way.

In a defence alliance countries promise to come to each others help if one member is attacked by an outside enemy. The idea is the alliance would be so powerful that wars would be prevented. An example of a defence alliance is N.A.T.O. (North Atlantic Treaty Organisation). This is mainly made up of allies of the U.S.A. and the U.K. from the second world war.