Lincoln’s original call for troops to serve 3 months was a mistake, as it turned out — nobody thought the war would last longer than that, and when it did, a lot of men who had been caught up in the initial enthusiasm were ready to go home. Many were willing to stay, as well, but the end of that 90 days was a time of great uncertainty for commanders. General Prentiss was worried about holding Cairo, Illinois, and Bird’s Point, Missouri across the river, with a reduced garrison.
From the OR, Series 1, Vol. 3, Part 1, p.406:
CAIRO, July 23, 1861.
CHESTER HARDING, Jr.:
Have but eight regiments here. Six of them are three-months’ men. Their time expires this week; are reorganizing now. I have neither tents nor wagons, and must hold Cairo and Bird’s Point. The latter is threatened. I have but two guns equipped for moving. Thus you see I cannot comply with request. Again, news of this morning changes policy of rebels in Kentucky. They are organizing opposite. Watkins is encamped with 2,000, 7 miles from Bloomfield. He has no cannon, and poorly armed. This may be the force you have heard from.
B. M. PRENTISS,