You never heard this side of the story before because there is no "this side of the story." It is a well accepted historical fact that the Union army burned Alexandria as they left the town in May 1864. There are numerous versions of the story in numerous eye-witness accounts, and while all the accounts may blame different segments of the Union army, everyone agrees that the Union army burned down Alexandria. Except . . . it seems the Louisiana Office of State Parks thinks the Confederate militia might have done it, as you will see if you read the displays at the visitor center of Forts Randolph and Buhlow State Historic Site, which will have its grand opening tomorrow morning (November 18, 2010) at 10 AM.
The display says that "some historians" think the Confederates might have set the fires to blame the Yankees. I've never read anything from any "historians" or eye-witnesses that suggests anything of the sort, and was wondering if anyone else had. They don't explain how the Confederate militia might have wandered through a town fully occupied by the Union army, or why they would want to burn down a town they were fixing to occupy themselves, except to say that they wanted to "blame" the Yankees. Why the burning of a few more buildings would get the Yankees in trouble, when they'd already burned down a good portion of the buildings between Natchitoches and Alex, is not explained.
And yes, there is a great book to be written about Alexandria during the War. I would love to write it, but finding the time for the research and writing involved is a problem.