Washington City and its suburbs, by the influx of Union volunteers, had during this month become a vast armed camp; the Potomac river had been crossed and the Virginia hills (including Arlington heights) which overlooked the Federal Capital, had been occupied and fortified by Union troops; the young and gallant Colonel Ellsworth had been killed by a Virginia Rebel while pulling down a Rebel flag in Alexandria; and General Benjamin F. Butler, in command at Fortress Monroe, had by an inspiration, solved one of the knottiest points confronting our armies, by declaring of three Negroes who had fled from their master so as to escape working on Rebel fortifications, that they should not be returned to that master--under the Fugitive Slave Law, as demanded by a Rebel officer with a flag of truce--but were confiscated "property," and would be retained, as "contraband of war." It was about this time, too, that the New Orleans Picayune fell into line with other unscrupulous Rebel sheets, by gravely declaring that: "All the Massachusetts troops now in Washington are Negroes, with the exception of two or three drummer boys. General Butler, in command, is a native of Liberia. Our readers may recollect old Ben, the barber, who kept a shop in Poydras street, and emigrated to Liberia with a small competence. General Butler is his son." Little did the writer of that paragraph dream how soon New Orleans would crouch at the very feet of that same General! And now, while the armed hosts on either side are assembling in hostile array, or resting on their arms, preliminary to the approaching fray of battle, let us glance at the alleged causes underlying this great Rebellion against the Union.