It’s a very complicated question. The Union was materially vastly stronger than the Confederacy; given the political will to fight to the end without doubt or hesitation, it could scarcely lose. The question then becomes whether it possibly could have lacked the will to fight.
Until Sumpter, the situation was confused. The lower south had declared it’s independence; the US federal government refused to recognize it, but had precious little power to do anything about it. Some in the north hoped it might somehow all blow over, others felt that allowing secession would be the lesser of two evils. As long as the upper south states still had a vote in congress, they could paralyze a war effort. As long as the US army held southern forts like Sumpter, it mocked southern claims of independence. Both sides thought that the other could be bluffed down, or would run scurrying with their tail between their legs if it came to a fight. Lincoln’s policy was to refuse to back down, but he wouldn’t (realistically, couldn’t) make the first move. It was up to South Carolina. (“Too small for a nation, too big for a madhouse”).
The question is, would prolonging this state of uncertainty for a year or two have helped the southern cause? I don’t know, it depends on just what the south was doing, or could do, to reenforce it’s position. In 20/20 hindsight, knowing that the South faced a long strategic war, it’s easy to say that the chance to prepare was squandered. But at the time many felt a bold stroke would be decisive.
Regarding British and French intervention, some other points. The Crimean War has been mentioned. The war had concluded five years before, and had been less than satisfactory to the British. The British had found themselves in a position similar to the current US position: possessed of enormous strategic strength (via it’s navy) but sharply limited in the number of boots it could put on the ground in hostile territory. This was the war of both the Charge of the Light Brigade and Florence Nightingale’s battle to establish decent care of the sick and wounded; it had revealed shocking deficiencies in the command and logistics structure of the British forces. The British had actually had to recruit foreign mercenaries to fill out the ranks; and when the French decided they had fullfilled France’s goals and unilaterally pulled out, the British were forced to seek a truce rather than a surrender from the Russians. The British were for the time being soured on foreign adventures. Selling another war would have been like trying to sell the US public on invading Iran in 1980, five years after the end of Vietnam.
Furthermore, British strategic doctrine was built around blockades, which it had a huge vested interest in upholding the legality of. To help the South, Britain would have had to do a face turn and declare the North’s blockade illegal and not binding on any neutral party- a potentially stupendous precedent. Indeed, during WW1, the British pointed to the American precedent of intercepting ships carrying war cargo to intermediate neutrals.
The South had the supreme misfortune of going to war at a time when an economic downturn had left Britain glutted with excess cotton and unsold cloth. If not a single fiber of Southern cotton had crossed the Atlantic after 1861, it would probably have taken a year to be felt. And the British were establishing plantations of long-fiber cotton in Egypt and India, meaning that there would never have been a complete cotton famine, even in a worst case scenerio. In the meantime, the British were getting rich not only running the blockade but selling both sides as much material as they could buy. The British would have been glad to see the upstart US broken into managable pieces, but they were content to wait and see if it would happen by itself. France, as has been noted upthread, was even less in a position to back the Confederacy.