The American civil war was started because the northern states wanted the southern states because of slavery issues and economic and political decision making.  The Trent affair happened because an American warship stopped a British mail ship and they arrested two confederate agents going to Britain for money to help the cause. Because of this affair Britain dispatched 14,000 troops just in case the fighting came to the BNA.  The St. Alban’s raid was performed by confederate soldiers using Montreal as their base and crossed the border to avoid getting in trouble for it.  The manifest destiny was a belief that the US was created by god to control North America.  A group of Irishmen created a brotherhood, called Fenian, to promote the liberation of Ireland from British control.  The Fenians believed that if they took control of British colonies they could hold them and use them as leverage to become liberated.

All of the summarized items would encourage a union of the colonies because they would benefit from it, like having a stronger military.