The number of armed men in town, including British, French, and Indians about quadrupled his own force.

This made heavy odds to face, even with the advantage of a surprise, and in spite of the fact that his own men were sure to fight to the last, since failure meant death by torture.
Moreover, if he made the attack without warning, some of the Indians and Vincennes people would certainly be slain, and the rest would be thereby made his bitter enemies, even if he succeeded.

On the other hand, he found out from the prisoner that the French were very lukewarm to the British, and would certainly not fight if they could avoid it; and that half of the Indians were ready to side with the Americans.

Finally, there was a good chance that before dark some one would discover the approach of the troops and would warn the British, thereby doing away with all chance of a surprise.
After thinking it over Clark decided, as the less of two evils, to follow the hazardous course of himself announcing his approach.

He trusted that the boldness of such a course, together with the shock of his utterly unexpected appearance, would paralyze his opponents and incline the wavering to favor him.