The Militia Act of 1862 allowed for members of organized militia units to be called into U. S. service for a period not exceeding nine months.
The state of Vermont was charged with raising ten regiments during 1862, but they were having a great deal of difficulty finding more than five regiments of men willing to serve for three years. Once the Militia Act was passed, they were able to raise five regiments of men willing to serve for nine months. These five regiments formed the Second Vermont Brigade.
My great-grandfather was one of those who chose to join a militia unit and eventually became a member of the 16th Vermont. I tend to believe that he found the militia/nine months approach to be a much more practicable alternative than signing up with the regular army for three years.