Whence "Uncle Sam"?

Sound reasonable to me:

So where did Uncle Sam originate? Nobody knows for sure, but it's likely . . . [that] the name was just a wiseguy expansion of the initials U.S.

It's worth noting that all the early references to Uncle Sam appeared in "peace" newspapers--that is, papers opposed to the War of 1812--and in every case the usage was derisive. This suggests Uncle Sam was dreamed up by critics of the government who simply wanted to personify the object of their scorn.

The Straight Dope