(CNN) -- The new Steven Spielberg blockbuster "Lincoln" focuses on the final months of the U.S. president's life as he tries to find a way to end the Civil War and unite the country. Now, the document Lincoln signed that officially started the war is up for sale, valued at nearly a million dollars.
The one-page document, signed April 19, 1861, authorized the blockade of Southern ports, which, under international law, was an act of war.
"The action was bold and with great risk," said Nathan Raab, vice president of The Raab Collection, which is offering the document for sale at $900,000.
"Lincoln was aware that the blockading of ports was an act of war. Some in his cabinet argued that a blockade would constitute recognition of the sovereignty of the Confederacy, something the North wanted to avoid. Lincoln was less interested in the legal definitions of 'war' than in victory, and he approved it despite the objections."
The Supreme Court later ruled the document as the official start of the Civil War, Raab said.
It was April 12, 1861, when Confederate forces opened fire on Fort Sumter in Charleston harbor, South Carolina, initiating the hostilities between North and South.
Three days later, Lincoln issued a proclamation that an insurrection existed, and he called out 75,000 men to put it down. Four days after that, on April 19, he ordered the Southern port blockade.
"I hereby authorize and direct 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 document states.
Lincoln's signature is at the bottom, "firm and bold, exactly as you'd like to see," Raab said.
The privately owned document has been on exhibit for several years at public institutions across the country, including the National Constitution Center and the Abraham Lincoln Presidential Museum and Library.
"It's been passed down to posterity quite well," Raab said. "It's survived the 150 years in good shape."
The Spielberg movie and the 150th anniversary of the Civil War have generated a lot of interest in the document and in Lincoln items in general, Raab said.
The document went on sale Tuesday night and will go to the first person who meets the asking price. Potential buyers can make contact with the seller through the Raab Collection's website, by phone, or in person.