Complex sentence: Harrison had persuaded Isaac Shelby, the Governor of Kentucky, to call up a brigade of 1,200 Kentucky militia under Brigadier General Green Clay, which followed Harrison down the Maumee but had not reached the fort before it was besieged.
Split into simpler sentences: Harrison had persuaded Isaac Shelby, the Governor of Kentucky, to call up a brigade of 1,200 Kentucky militia under Brigadier General Green Clay. Clay's brigade had followed Harrison down the Maumee, but had not reached the fort before it was besieged.