An international alliance is a formal group of mostly sovereign states that agree to work together. They can be military alliances like NATO, political unions or groups that focus on resources or culture. They can also be a combination of all these. The objective is to achieve common goals, but the approach varies from country to country. Most alliances of states have some tensions based on the fact that members are often forced to relinquish some part of their sovereignty. Some also face the problem that some member states benefit more than others, which can create dissatisfaction.
For example, Western European countries formed a military alliance in 1949 after World War II, when they realized that the continent was too dangerous for them to face each other alone. This alliance created the Brussels Treaty, which said that if one of its members was attacked, the other countries would come to their aid. This became known as collective security. In the 21st century, the alliance has evolved to meet new threats. The September 11th attacks prompted the first-and only-time that Article 5 was invoked, and NATO went on to lead an international force in Afghanistan.
Moreover, there are alliances that deal with issues like economics and global governance. They are called strategic partnerships. They are less formal and do not require a treaty, but they help the partner nations build relationships that can be useful for them. This kind of partnership is especially important for smaller states, which can find it difficult to negotiate with larger countries.