After you left, I ascertained that less than twenty thousand unorganized men, without a single field battery, were all you designed to be left for the defence of Washington, and Manassas Junction; and part of this even, was to go to Gen. Hooker's old position. Gen. Banks' corps, once designed for Manassas Junction, was diverted, and tied up on the line of Winchester and Strausburg, and could not leave it without again exposing the upper Potomac, and the Baltimore and Ohio Railroad. This presented, (or would present, when McDowell and Sumner should be gone) a great temptation to the enemy to turn back from the Rappahanock, and sack Washington. My explicit order that Washington should, by the judgment of all the commanders of Army corps, be left entirely secure, had been neglected. It was precisely this that drove me to detain McDowell.

What It Means

This is a placeholder explanation generated by the low-overhead model. It interprets "After you left, I ascertained that less than twenty thousand unorganized men, without a single field battery, were all you designed to be left for the defence of Washington, and Manassas Junction; and part of this even, was to go to Gen. Hooker's old position. Gen. Banks' corps, once designed for Manassas Junction, was diverted, and tied up on the line of Winchester and Strausburg, and could not leave it without again exposing the upper Potomac, and the Baltimore and Ohio Railroad. This presented, (or would present, when McDowell and Sumner should be gone) a great temptation to the enemy to turn back from the Rappahanock, and sack Washington. My explicit order that Washington should, by the judgment of all the commanders of Army corps, be left entirely secure, had been neglected. It was precisely this that drove me to detain McDowell." in the context of "Abraham Lincoln, 9 April 1862 telegram to McClellanhttps://quod.lib.umich.edu/l/lincoln/lincoln5/1:410?rgn=div1;view=fulltext" to mean that wisdom is timeless.