Inspired by the "German-Style Unification of the US" thread...
Say the Constitutional Convention fails and the Articles of Confederation drag on for a bit longer before the whole things falls apart. The various colonies go to war with each other over land claims and the like.
The wars produce a stream of refugees to the southwest. They settle in Texas under the nominal protection of Spain and eventually grow too big for the Spanish to control, establishing a large state of their own in the Gulf Coast.
Eventually, it is this uber-Texas that forcibly reunites the United States. I'm thinking a combination of Texas-as-Prussia mixed with Battletech's Clans.
How could this scenario come about? The above is a very vague outline, subject to change.