How did he do it?

Well, by now you know that George Rogers Clark was a twenty-four year old Virginian who defeated a much more experienced British commander withe nearly triple the men available to him. But how did he do it? a big cannon? Sheer luck? No, Ol' George was a master of psychology. He talked him into it. For starters, he talked the men in his command into coming with him, despite the fact that he half of them. Then, he used the fact that residents of Kaskaksia thought little better of them than the Indians to his advantage, and talked them into joining. Then our story begins. He sent Leonard Helm to Vincennes to staff the fort. Captain Helm wound up surrendering the Henry Hamilton a little before Christmas, 1778, and word reaches Colonel Clark in late January via Francis Vigo. Back then, Armies stayed put in winter,so I'm sure that news hit him hard. So what does he do? He moves his army in winter!

They made it a though a flood, sieged the fort, and forced its surrender. The rest is history.