Unfortunately, one single alliance is usually not strong enough to win the server. Therefore, people can and  will use diplomacy to prevent future wars or increase their chances of winning. There are three possibilities: confederation, non-aggressive pact and war. 

Confederation

A confederation is a strong bond between two or more alliances. In this case, alliances help each other and cooperate between themelves more. Usually, they help each other by sending reinforcements or resources if needed. Also, they can manage massive attacks together,  share wars and fight their common enemy. On servers that start after 15.09.2019, a new confederacy system is used which further increases the cooperation between alliances and restricts resource and reinforcement sharing to the confederation members only.

Non-Aggressive Pact

A non-aggression pact (NAP for short) is used when two alliances do not want to attack each other, but they do not want to help each other either. It's commonly used between strong alliances which would damage  eachother badly and they want to avoid such an event. These are limited to 3 per alliance.

War

War is a heavy struggle between two alliances. Alliances on the confederation list are usually standing behind alliances in wars.