An alliance is a formal agreement between two or more groups to work together toward a shared goal. When countries form an alliance, they promise to support and defend each other, especially during wars or conflicts. When two students form an alliance in a game of capture the flag, they agree to help each other win instead of competing against one another.

Alliances matter because they make groups stronger through cooperation. NATO (the North Atlantic Treaty Organization) is a famous alliance where democratic countries promise to protect each other if any member is attacked. During World War II, the United States, Britain, and the Soviet Union formed an alliance to defeat Nazi Germany, even though these countries had very different governments and didn't always agree on other issues.

The word can apply to smaller situations too. Two businesses might form an alliance to share resources, or several schools might create an alliance to compete in a regional science fair. In strategy games like Risk or Diplomacy, forming alliances with other players can mean the difference between victory and defeat.

An ally is a partner in an alliance. Countries that are allies support each other, just as friends who are allies stand together when facing a common challenge. The key to any alliance is that everyone involved benefits more by working together than they would by working alone.