Dissolving coalitions
An AI nation will only create a coalition if it has the Outraged relations attitude. Once a coalition has been formed, nations with the Rival, Hostile or Threatened attitudes are likely to join it. If no nation in the coalition has the Outraged attitude, the coalition will dissolve. A nation with this attitude will switch to another one if its opinion becomes positive, so if there is only one Outraged nation in the coalition, placating it is an effective way to get rid of the coalition.
Outraged only leave at positive relations.
Rivals and Threatened leave at -99 or better.
Hostile nations leave regardless of relations.
Nations won't always immediately leave upon hitting those breakpoints as they are sometimes out of diplomats for a brief period
EDIT : il semble à la lecture du paragraphe au dessus, que si tu améliores tes relations uniquement avec les "outraged", l'ensemble de la coalition tombe (plutot que de cibler tout le monde)
et en addition :
Coalition Warfare Mechanics
The coalition call-to-arms is an automatic one and will result in all members without a truce with the target and not currently at war with another coalition member instantly being part of the war. There is no option to decline.
Each coalition has a war leader. If the coalition is attacking, the war leader is the country that declared war. If the coalition is defending, the war leader is the most powerful nation in the coalition (determined how?)
The war leader always negotiates for the entire coalition; unlike alliances, countries in a coalition cannot make a separate peace. This means that if countries opt out of allowing the leader of a coalition to negotiate for them they will not be a way to gain provinces from them in that specific war.
Occupying all of the provinces of a member of a coalition will allow a country to ask for that member as a vassal in the peace deal.
War leaders may call non-coalition allies into the war, but these allies are not bound by the coalition rules - they may decline the call, and can sign a separate peace with the enemy.
Source : http://eu4wiki.com/Alliance