The Union was the main country that had years of peace. The government had an army of 800,000 always ready to defend.
That force would be needed when on February 1, 1861, the South declared secession from the Union. Abraham Lincoln, that was just elected as president, responded by attacking Richmond. However, the attack was thwarted and the Union was sent back to where it started. They then raised their force to 1,100,000 and declared war on the South.
So far, the Union has fought in 3 campaigns.