In thinking a bit more on this, and being a historian of the Napoleonic era, I should mention here that the influence of the French Grande Armee on other armies of the world was profound, in particular the US Army. Usually, when you have an army that dominates an era, other armies tweak their own to emulate them. This starts as far back as the ancient Greeks, where some of their foes (including other Greek city-states) tried to emulate their armor and tactics, or, even hired Greeks to fight for them (as the Persians did - see Xenophon's March of the Ten Thousand). The greatest army in military history (IMHO), the Roman Army, was copied in many aspects by its enemies. So it goes with the US Army emulating some of Napoleon's army (as did the Mexican Army of Santa Anna).

With Beauregard having been in the US Army there may have been some influence in his choice of flag shape. The drill manual used by the Union and Confederacy was "written" by William J. Hardee, who studied for some time in France. He basically re-wrote the 1840s French manual to reflect the minie bullets and their extended range on the battlefield. West Point had a Napoleon Club to which many cadets belonged.

Greg Biggs