British infantrymen occupy a shallow trench in a ruined landscape before an advance on the first day of the Battle of the Somme, on July 1, 1916. One of the most vicious conflicts of World War I, the battle raged across northern France. By its close on Nov. 18, millions of men had battled through the mud and blood of the treacherous trenches on both sides. (AP/PA)

Ninety-nine years ago today, the world was a year out from an armistice that would bring (some of) the fighting to an end in the first truly global conflict. The United States commemorates Nov. 11 as Veterans Day, but in Europe it’s Armistice Day, marking the end of World War I, and four years of bloody fighting.

On Nov. 11, 1917, the Central Powers, led by Germany and Austria-Hungary, appeared to hold the strategic advantage. Russia — part of the Triple Entente with France and Britain — was on the verge of a defeat that would gut the population, production and territory of its western empire.

Serbia was crushed, satisfying the goals that drew Austria-Hungary to war in the first place. Austria-Hungary had also survived Italy’s best shot along the Isonzo River, and the Ottoman Empire had fended off the British and French at Gallipoli.

Farther west, German U-boats continued to sink merchant shipping critical to Entente resupply. Russia’s defeat promised the redeployment of 54 German divisions to the Western Front, divisions that might yet smash the British and French armies struggling to evict Germany from Belgium and France.

Why did this horrific and costly war last even another year?

Here are three reasons that November 1918, not November 1917, would finally see the armistice that gave us today’s holiday.

1. Belgium had not yet been restored to its buffer state status.

World War I started over Austro-Hungarian fears of rising Serbian power in the Balkans, and it expanded because Germany was happy to see resurgent Russian influence stifled in Eastern Europe. Germany risked a wider war by issuing the now-infamous “blank check” of support for Austria.

But Germany’s eastern ambitions required that it fend off Russia’s western friends — France and Britain — and that meant invading Belgium.

The quintessential buffer state, Belgium’s location allowed whatever member of the Franco-British-German triad that controlled it to menace the others. No, Britain was not defending “gallant little Belgium” to preserve the principle of right over might. The British Empire joined the war because whichever power held Belgium would be tempted to try to undermine the others as great powers.

No power holding Belgium could be trusted to abide by a compromise peace. Letting Germany stay in Belgium would only guarantee its ability to wage another war against the British or the French. Therefore, the Entente would not cut a deal with the German Empire in 1917 unless it could restore this critical buffer state.

2. German leaders knew it was all or nothing — they’d be punished if they lost

The desire to restore Belgium and quash a German bid for hegemony precluded almost any outcome but capitulation. The Entente and its American “associate” — explicitly not an ally — had more money, more people and more production, nearly ensuring that they would win out in the long-run attritional contest that the war had become.

These facts weren’t lost on German leaders, who believed by late 1916 that they would lose the war unless something changed. Germany might have yielded sooner if the issue were simply accepting defeat and a fate hemmed in between rival great powers.

But Germany’s leaders faced a restive population and few legal restraints on the treatment of deposed leaders. Their great fear was that anything short of a victory that produced new land, power and bread would mean not just political but personal doom — that is, exile, jail or death.

So Germany’s leaders kept up the fight in desperate gambles — from 1917’s unrestricted submarine warfare to last-gasp offensives in 1918 — that preserved at least some chance of victory. If compromise meant near-certain peril, fighting on a bit longer for victory made cynical, if costly, sense — what political scientists call “gambling for resurrection.”

3. U.S. intervention ensured the war lasted only one more year

Belgian geography and the self-preservation instincts of Germany’s leaders explain why the war didn’t end in 1917. But geography doesn’t change, and leaders can hang on for a long time. So why did the war continue for only one more year?

Enter the Americans.

The Entente fought as long as it did in part because of confidence that eventually America would join the fight. Germany’s renewed policy of unleashing its submarine fleet on neutral shipping proved the last straw, and the United States declared war on Germany in April 1917.

It would take some time before the small, inexperienced U.S. Army could make its presence felt. In fact, part of Germany’s rationale for unrestricted submarine warfare was closing Atlantic sea lanes before U.S. troops could arrive in force. But the U-boats were less effective than promised, prompting one last bid for victory in 1918. Germany’s spring offensives, leveraging the divisions freed up from the East, made dramatic gains in their first few weeks.

But armed with the promise of U.S. troops shoring up their reserves, France and Britain mounted a more tenacious defense and a more vigorous counterattack than they could have hoped for without thousands of doughboys pouring into Europe every month.

Once U.S. support turned tangible, it helped force an end to the war much sooner than anticipated. Indeed, before the Germans exhausted their reserves in the 1918 spring offensives, even the U.S. Army anticipated that the war might last into 1919. But once Kaiser Wilhelm II learned that his army would no longer fight for him, the writing was finally and indelibly on the wall. Germany sued for peace, producing the armistice of Nov. 11, 1918, that we commemorate today, and each year.

Scott Wolford is associate professor of government at the University of Texas at Austin. He is writing a textbook that uses game theory to teach the politics of the First World War. Follow him on twitter at @thescottwolford.