How and why did the British North American Colonies (Canada) had a role in the United States Civil War?

The American Civil War (1861–65) was fought between the northern (Union) states and the southern (Confederate) states, which withdrew from the United States in 1860–61. The war left cities in ruins, shattered families and took the lives of an estimated 750,000 Americans. The war also involved those living in what is now Canada, including roughly 40,000 who joined the fight. The war played a significant role in how and when Canada became an independent country.