January and February were added later to push each numbered month to 2 off

The reason for this being that at the beginning of the year, some government officials changed posts, including the ones responsible for the military. March is when the weather becomes nice, so you can go to war again (hence the name). But since March also marked the new year, the people responsible for war management just took office, which was not ideal. So they pushed the two months in to give them some time to prepare before sending out the military everywhere.

Fun times!