Was a declaration written for the Civil War?

Technically, the answer to this question is no. Because Lincoln did not recognize the Confederate States as a nation, he could not legally ask Congress to declare war against them. Doing so would constitute recognition in the eyes of European nations, in particular. But Lincoln did issue what amounted to a declaration of war in the aftermath of the attack on Fort Sumter by Confederate forces. He ordered a blockade of Southern ports, declared the southern states in a state of rebellion, and called for 75,000 men to crush the rebellion. This action, which was approved by the Supreme Court and a special session of Congress (also called for in Lincoln's Proclamation) essentially marked the beginning of the Civil War. Many of Lincoln's actions while President, including the issuance of the Emancipation Proclamation, were framed as wartime measures. Others, like suspending the writ of habeas corpus, were justified by the existence of a state of insurrection (the formal legal status of the conflict from a Union perspective). But in any case, the distinction between "war" and "insurrection" was only a legal one. The armies of the Union and the Confederacy waged brutal warfare against each other for four years with hundreds of thousands of men killed as a result.