What if the Confederate army had not fired on Ft. Sumter?
First, the windup.
South Carolina left the Union on December 20, 1860. South Carolina sent commissioners to Washington to negotiate the withdrawal of U.S. army forces and accounting for federal property in South Carolina. In the meantime, Ft. Sumter was unoccupied. Charleston officials assured the senior U.S. Army officer in the area, Major Robert Anderson in Fort Moultrie on the mainland, that, while the commissioners were discussing terms with Washington, they would not make any hostile move in Charleston harbor. Moultrie was too big for Anderson to defend, so, on Christmas Eve, Anderson spiked his guns and burned the gun carriages in Moultrie and moved his entire command to Ft. Sumter, where they were much less vulnerable (and thus much more threatening to Charleston).
On December 27, 1860, the bark Copernicus of the German city of Bremen, arrived at Charleston. The master of the ship proceeded to the customs house and attempted to pay the U.S. customs. The customs officials said that there were no U.S. customs officials in Charleston. They were now the customs officials of the state of South Carolina. The master of the Copernicus attempted to pay them, but they refused to accept any money from him. Now he was worried. If someone later came to him and accused him of not paying the customs duties, he was personally liable for a huge fine, so he wrote to the Bremen consul in Charleston, Mr. R. Schleiden. Schleiden wrote to U.S. Secretary of State Black asking what he must do to protect his citizens from liability. Black said he would discuss the matter with President Buchanan. The Charleston Courier of February 5, 1861 printed Schleiden’s correspondence with the U.S. Secretary of State. Merchant ships, both American and European, continued to enter and depart Charleston despite the presence of the Union garrison in Sumter.
On February 8, 1861, the Confederate Congress adopted the low federal tariff of 1857, and then expanded the list of items that could enter the Confederacy duty-free. The Confederate Congress adopted a law allowing duty-free transition of Confederate territory for goods from a foreign port destined for the United States. So British cotton cloth could land in Charleston, take a train to say, Richmond and completely avoid the high Morrill tariff.
The United States adopted on March 3, a large increase in federal tariffs, called the Morrill Tariff, to take effect 1 April. In late March 1861, a group of New York businessmen visited Lincoln and assured him of financial support for the federal government if a war broke out, the unstated premise was that Lincoln must not allow secession to happen peacefully. April 5th, Lincoln decides to send a military expedition to resupply Ft. Sumter (and, incidentally, Ft. Pickens outside Pensacola). When the federal relief expedition arrived off the coast of Charleston, the bombardment started and the war came.
As Jeff Davis was debating whether to bombard Sumter before the arrival of the Union military expedition, Confederate Secretary of State Robert Toombs urged the Confederates avoid firing the first shot. "Mr. President, at this time, it is suicide, murder, and will lose us every friend at the North. You will wantonly strike a hornet's nest which extends from mountains to ocean, and legions, now quiet, will swarm out and sting us to death. It is unnecessary; it puts us in the wrong; it is fatal… So long as the United States neither declares war nor establishes peace, the Confederate States have the advantage of both conditions."
So, what if the Confederates do not fire on Fort Sumter and just ignored it?