To match the Confederates who already had 100,000 men in their army. Virginia had already been "invaded" by a foreign army when the Governor invited Confederate troops from Texas and other deep south states into his state over a month before they voted to seceed.
I'll have to check that chronology. Nevertheless, in retrospect, we can clearly see that the north was the invader. The south wanted to be left alone and they stated as much many times. Jackson and Lee both fought to defend their homes - no other reason. The north committed many atrocities and I tire of watching people paint the north white and south black. The north was the aggressor, and Lincoln intimated in his 2nd inaugural that the judgmeent of God had fallen on the ENTIRE nation, not just the south.