league 普通用词，指为了共同的利益而建立的地区性、全国性或国际性的联合组织，也指民间或半官方组织的联盟。
federation 泛指用条款固定下来的一切联合，尤指联邦政府。
union 普通用词，指由若干个体结成的一个统一的整体。强调其永久性。
alliance 普通用词，指对双方均有利的任何联盟。
参考例句
1. How do we form an alliance? 那我们怎样形成联盟呢？
2. That is because it feels threatened by the Wildrose Alliance, a more conservative fringe party. 那是因为他感觉到野玫瑰联盟（一个更保守的边缘党派）的威胁。
3. The Communist enterprise economy system includes Communist enterprises, Communist groups, Communist staff and the Communist enterprise alliance. 共产企业经济体系包括共产企业、共产团体、共产人员和共产企业联盟。
英英解释
n.
1. the state of being allied or confederated
2. a connection based on kinship or marriage or common interest
3. an organization of people (or countries) involved in a pact or treaty
4. a formal agreement establishing an association or alliance between nations or other groups to achiev
5. the act of forming an alliance or confederation
