Frank van der Linden, Lincoln, The Road to War, Fulcrum Publishing, Golden, Colorado, 1998, pg. 279-280:
On Sunday, April 14, Lincoln met with his cabinet and drafted a proclamation, to be telegrahed to the nation the next day. He could not declare war—under the Constitution, only the Congress could do that—but, in effect, he announced that a war was going on because the federal laws were being obstructed in the seven seceded states by “combinations too powerful to be suppressed” by the courts and the marshals. Therefore, he called for the governors to provide seventy-five thousand militia “to suppress said combinations and to cause the laws to be duly executed.”As the slim legal basis for his policy, Lincoln relied upon a 1795 law he interpreted as giving him this authority, which amounted to summoning a “posse comitatus” of record size—seventy-five thousand men—to enforce the federal laws. He also called upon “the persons composing the combinations aforesaid to disperse and retire “drawn peaceably to their respective abodes within twenty days.” He pledged that “the utmost care will be observed ... to avoid any devastation, any destruction of, or interference with, property, or any disturbance of peaceful citizens of any part of the country. ” Those assurances were to prove grimly ironic as the ensuing civil war brought ruin upon the South and the loss of its “peculiar” property, the slaves that had an estimated value of $4 billion, all reduced to zero.
Lincoln also called Congress into special session, but delayed the opening date until the Fourth of July. Why did he not summon the lawmakers to come at once and help him cope with the great crisis? Carl Schurz, the brilliant young German devoted to the Republican cause, provided the answer in a letter to Lincoln in early April: “Some time ago, you told me you did not want to call an extra session of Congress for fear of reopening the compromise agitation.” Schurz suggested that, after a show of force to defend the forts, Lincoln should call Congress back and then “the enthusiasm of the masses will be great and overwhelming,” and “Congress will be obliged to give you any legislation you may ask for.” On the other hand, Schurz darkly prophesied, if Lincoln seemed indecisive, “we shall be beaten in most of the Northern states in the fall elections and your administration will be at the mercy of Democratic demagogues.”
Endnoted to "'Some time ago': Carl Schurz to Lincoln, April 5, 1861, Lincoln Papers, LC."