What it is: When people, groups and states join together and co-operate to achieve a shared purpose, this can be called an alliance. Sometimes it may be an informal arrangement; sometimes there may be a signed agreement.
What it means: Some say that the political alliances made before 1914 helped to cause the First World War. The war was certainly fought by two alliances: the Central European Powers (Germany, Austria-Hungary, Turkey and Bulgaria) on one side and, on the other, the Triple Alliance or Entente (Britain, members of its empire, France, Russia and other allies; America joined the war in 1917). In the Second World War the Axis Powers (Germany, Italy and Japan, and later some eastern European countries) went to war with the Allies (49 countries including Britain, France, later the Soviet Union, and later still, America). Some members of alliances may change sides (as Bulgaria, Finland, Hungary and Italy did in the Second World War). In 1949 western European countries and the USA formed the North Atlantic Treaty Organisation (NATO), a military alliance, because they believed the Soviet Union (a political alliance) posed a threat. As a result the Soviet Union formed a military alliance of its own, the Warsaw Pact, with eastern European communist countries. After the Cold War ended, there seemed to be no reason for NATO to continue, but by then it had become so complex an organisation, so much a part of the scenery (and the arms trade), that instead of disbanding its leaders looked for a new role. By 2000 former Warsaw Pact countries had become members of NATO themselves, or had applied to join. NATO had also sent troops into the Balkans to try to stop ethnic conflicts there. It also had its own problems, such as the dominance of American influence over it.
Think about it: Alliances are not necessarily a good thing in themselves. They can be helpful when the common purpose is a benign one, but they are more often created to exert power. There are risks for individual members, some of whom may not get much say in what goes on, and the chance of being caught up in unwanted situations by the actions of just one member. There is also the risk that people or groups outside an alliance may become resentful or hostile: the very existence of an alliance can create distrust and start a conflict. Even local events can get out of hand when rival or hostile groups and gangs are formed. On the other hand, working as a team, co-operating to achieve something worthwhile, benefits everyone. So what makes some alliances breed conflict and others benefit the people? For a start, what about the reasons for uniting?