The Revolutionary War on the American continent is not yet complete. The Puckheads to the north are still loyal to the British crown and we can't have that going on with Manifest Destiny as one of the pressing issues of the upcoming presidential election.
And you know, the third time is always the charm! You know if it wasn't for Benedict Arnold (Yup, the traitor) losing a key battle in the Revolutionary War, Canada would probably be a part of the U.S. But the American Spirit encouraged us in the War of 1812 and we came up just short.
But in the past 200 years, our military forces have grown and become the best in the world, so let's give it another go! And besides, Alaska gets the shaft for not being included in the 48 contiguous states, so let's help them out & make them feel welcome!