PHILADELPHIA — A document signed by President Abraham Lincoln ordering Union blockades of Confederate ports, marking the official start of the Civil War, is for sale.

The Raab Collection in Philadelphia said Tuesday it is selling the document, which it calls one of the most important in U.S. history. The asking price is $900,000.

Lincoln’s proclamation is dated April 19, 1861 — a week after the first shots of the conflict were fired at South Carolina’s Fort Sumter. After the Civil War ended in 1865, the U.S. Supreme Court in an opinion ascribed Lincoln’s April 19 blockade order as the official beginning of the war.

“This action was bold and with great risk,” said Nathan Raab, vice president of The Raab Collection. “Lincoln was aware that the blockading of ports was an act of war.”