Interestingly, I think in the Prize cases the Supreme Court ruled that the war began with this April 19th presidential proclamation some days after Fort Sumter, even though the Constitution says that Congress is the one that declares war, not the President. A minority of the court felt that the war didn't start until July 1861, when Congress acted. Most people, of course, consider the firing on Fort Sumter to be the start of the war, but not the Court.
Jefferson Davis recognized Lincoln's April 15, 1861, proclamation calling for 75,000 troops as the declaration of war. (Source: Message of Davis to the Confederate Congress on April 29, 1861)
Here's the April 19th blockade proclamation:
BY THE PRESIDENT OF THE UNITED STATES OF AMERICA:
A PROCLAMATION:
Whereas an insurrection against the Government of the United States has broken out in the States of South Carolina, Georgia, Alabama, Florida, Mississippi, Louisiana, and Texas, and
the laws of the United States for the collection of the revenue cannot be effectually executed therein comformably to that provision of the Constitution which requires duties to be uniform throughout the United States:
And whereas a combination of persons engaged in such insurrection, have threatened to grant pretended letters of marque to authorize the bearers thereof to commit assaults on the lives, vessels, and property of good citizens of the country lawfully engaged in commerce on the high seas, and in waters of the United States: And whereas an Executive Proclamation has been already issued, requiring the persons engaged in these disorderly proceedings to desist therefrom, calling out a militia force for the purpose of repressing the same, and convening Congress in extraordinary session, to deliberate and determine thereon:
Now, therefore, I, Abraham Lincoln, President of the United States, with a view to the same purposes before mentioned, and to the protection of the public peace, and the lives and property of quiet and orderly citizens pursuing their lawful occupations, until Congress shall have assembled and deliberated on the said unlawful proceedings, or until the same shall ceased, have further deemed it advisable to set on foot a blockade of the ports within the States aforesaid, in pursuance of the laws of the United States, and of the law of Nations, in such case provided. For this purpose a competent force will be posted so as to prevent entrance and exit of vessels from the ports aforesaid. If, therefore, with a view to violate such blockade, a vessel shall approach, or shall attempt to leave either of the said ports, she will be duly warned by the Commander of one of the blockading vessels, who will endorse on her register the fact and date of such warning, and if the same vessel shall again attempt to enter or leave the blockaded port, she will be captured and sent to the nearest convenient port, for such proceedings against her and her cargo as prize, as may be deemed advisable.
And I hereby proclaim and declare that if any person, under the pretended authority of the said States, or under any other pretense, shall molest a vessel of the United States, or the persons or cargo on board of her, such person will be held amenable to the laws of the United States for the prevention and punishment of piracy.
In witness whereof, I have hereunto set my hand, and caused the seal of the United States to be affixed.
Done at the City of Washington, this nineteenth day of April, in the year of our Lord one thousand eight hundred and sixty-one, and of the Independence of the United States the eighty-fifth.
ABRAHAM LINCOLN
By the President:
WILLIAM H. SEWARD, Secretary of State
Just before the war ended, Lincoln waved a magic wand and changed the four year old blockade (an international act) into a closing of the ports (a national act).