If twentieth-century Germany had a tombstone, it would say "This is What Happens to Those Who Fight on Two Fronts". Much as kung-fu movies make fighting multiple opponents look easy, it's generally better to defeat your enemies one at a time.

That was the idea behind Germany's Schlieffen plan, which called for concentrating on France in the opening days of the conflict while keeping weaker forces in the East. The key was to defeat France quickly while vast and underdeveloped Russia still mobilized, and then transfer forces by rail to settle accounts with the Tsar.

However, Russia did attack into East Prussia in August 1914, only to be surrounded and annihilated at the Battle of Tannenberg. They lost 170,000 men to just 12,000 Germans in one of history's most famous battles of encirclement. Yet the Russian advance also frightened German Army Chief of Staff Helmuth von Moltke into transferring three corps from France to East Prussia. They arrived too late for Tannenberg, while depriving the Western offensive of vital troops at Germany's best time to overcome France and possibly end the war.

From then on, Germany had to spread its forces between West and East, while supporting its Austro-Hungarian and Turkish allies. Just what Germany could have accomplished--had it been able to concentrate on just one front--became painfully clear in 1918. After forcing the new Soviet government to sue for peace, the Germans quickly transferred 500,000 troops to France. They also unleashed innovative new stosstruppen (stormtrooper) infiltration tactics--an early form of blitzkrieg without the tanks--that enabled them to break the trench-warfare deadlock.

Kaiserschlacht ("Kaiser's Battle") offensives shattered several British armies and compelled British commander Douglas Haig to warn his troops that their backs were "to the wall." After four years of unrelenting combat and economic blockade, Germany still had the strength to achieve more in weeks than four years of bloody Allied offensives at the Somme, Passchendaele and Chemin des Dames.

Ideally, Germany could have found diplomatic means to have fought against Russia alone without war with France, or vice-versa.