With war clouds hanging heavy over Washington in early April 1861 and the budding Confederate States of America a reality, Union General-in-Chief Winfield Scott developed a plan for the execution of the onrushing war. Scott's concept, later dubbed the Anaconda Plan, consisted of the blockade of the Southern seaports and control of the Mississippi River. This, he believed, would strangle the South by preventing it from exporting its crops for currency, preclude its receiving needed supplies and weapons to support its war effort, and isolate the western from the eastern sections of the Confederacy.
Lincoln was aware that the blockading of ports was an act of war. In fact, since an act of war is, by implication, taken against another state, some in his cabinet argued that a blockade would constitute a tacit recognition of the sovereignty of the Confederacy, something the North wanted to avoid. Lincoln was less interested in the legal definitions than in the military utility of the plan, and he approved it despite the objections.
On Friday, April 12, 1861, Confederate forces opened fire on Fort Sumter in Charleston harbor, initiating hostilities between the North and South. Lincoln immediately began moving to meet the crisis head on. The U.S. Army had less than 800 officers and only some 14,000 enlisted men, yet the federal government needed to mobilize for war. The only law in existence permitting the raising of additional troops was the Militia Act of 1792, which empowered the president to call out the militia to suppress insurrection. Using this law, on April 15, Lincoln issued a proclamation declaring that an insurrection existed, called out 75,000 men to put it down, and convened a special session of Congress for July 4.
After the war, the Supreme Court issued an opinion fixing the exact dates on which the war began and ended. It held: “...The proclamation of intended blockade by the President may therefore be assumed as marking the first of these dates, and the proclamation that the war had closed, as marking the second.'
On April 19, Lincoln issued his proclamation blockading Southern ports. It provided that "a competent force will be posted so as to prevent entrance and exit of vessels" from the ports of the states in rebellion. Then, to make the proclamation official, he signed this document, authorizing "the Secretary of State to affix the Seal of the United States to a Proclamation setting on foot a Blockade of the ports of the States of South Carolina, Georgia, Alabama, Florida, Mississippi, Louisiana, and Texas." The seal was affixed to the blockade proclamation, which was announced that day. It was a de facto declaration of war by the Union against the Confederacy.
By the end of 1861, over 250 warships were on duty, with 100 more under construction. By 1865, some 600 ships were patrolling the Confederate coastline. Moreover, as the war progressed, the Union also intensified the blockade's effectiveness by capturing or sealing off a growing number of Southern ports. The storied blockade-runners were increasingly stymied. In the blockade’s first year, their chance of capture was one in ten. By 1864, the odds had become one in three, and by 1865, one in two.
Strategically, the blockade was decisive. It limited both the import of military and other needed supplies and the export of income-producing cotton. "The blockade reduced the South's seaborne trade to less than a third of normal. And of course the Confederacy's needs for all kinds of supplies were much greater than the peacetime norm. As for cotton exports,....the half-million bales shipped through the blockade during the last three years of war compared rather poorly with the ten million exported in the last three antebellum years...[And] the blockade was one of the causes of the ruinous inflation that reduced the Confederate dollar to one percent of its original value by the end of the war." (James McPherson in Battle Cry of Freedom). The authoritative Historical Times Illustrated Encyclopedia of the Civil War states, “Historians generally agree that the blockade, with more than 600 ships, not the force of Union arms, finally brought about the downfall of the Confederacy.”
The U.S. Supreme Court Ruling on the War’s Official Commencement
After the war, the Supreme Court issued an opinion fixing the exact dates on which the war began and ended. It held: “...The proclamation of intended blockade by the President may therefore be assumed as marking the first of these dates, and the proclamation that the war had closed, as marking the second.'' Thus, according to the Supreme Court, Lincoln’s signature on this order sealing the imposition of the blockade marked the official beginning of the Civil War.