Germany's involvement in World War One was significant in both Western and Eastern Europe. The Germans had the strongest military of their alliance, and indeed perhaps the rest of Europe. As such, their alliance, which included Austria-Hungary and Ottoman Turkey, almost entirely depended on German military support.
Upon the outbreak of military hostilities in 1914, with the assassination of archduke Ferdinand, Germany had allied itself with Austria-Hungary and Ottoman Turkey. They pledged military support to each other. When the war in the Balkans began, Germany was committed to Austria-Hungary against the Serbs.
However, Russia also had interests in the Balkans. The Russians were committed to Serbia, so any war with them could likely bring Russia in. Germany and Russia remained acutely aware that their alliance obligations could result in war between them. When Russia mobilized its army in Eastern Europe, it was enough to prompt a German declaration of war on them.
Britain and France had established the Triple Entente with Russia. When Germany declared war on Russia, Britain and France mobilized their armies. Germany hoped to defeat France first before Russia, in accordance with its Schlieffen Plan. As such, the German king also declared war on the French.
The Schlieffen Plan aimed to quickly defeat France before Russia. So Germany also needed to invade Belgium. Britain pledged to defend Belgium if Germany violated the country's neutrality. As Germany did so, Britain joined the war.
Germany's plan to defeat France initially went well as its troops swept into France, breaking French lines. However, as the BEF joined France's army they were able to halt the onslaught at the First Battle of the Marne in 1914. It was here that the Entente halted the German advance, and both sides began to dig in. The Schlieffen Plan had failed to win a quick victory.
Germany did not have a clear plan b after defeat at the First Battle of the Marne. However, in the East its army won some notable victories against Russia such as at Tannenberg as the Russians retreated from Poland. As such, the Eastern Front soon became Germany's most successful in the war. Victory against Romania in 1916 further boosted Germany.
However, the war in the West dragged on. By 1916, the war had not been won by either side; but Wilhelm still declared a German victory and offered to negotiate peace with the Entente. This victory declaration was somewhat 'hot air,' for although Germany were winning in the East, the Western Front was indecisive. Peace negotiations fell through and war continued.
In 1917, the tsar abdicated the throne in Russia; and Germany was on the brink of one victory. The Bolsheviks soon began negotiation with Germany, and reluctantly agreed to the Treaty of Brest-Litovsk. This effectively ended the war in Eastern Europe and broke the Triple Entente alliance.
With this, a German victory in the war may have seemed more likely. However, the war in the West was not going so well. Unrestricted submarine warfare not only failed to break the Royal Navy blockade, but also convinced America to declare war on the Germans.
Germany attempted a last advance in the West in 1918. The Kaiserschlacht started well and broke through Entente lines. However, it was eventually halted at the Second Battle of the Marne that enabled an Entente counter-attack now called the Hundred Day Offensive. This delivered a decisive Entente victory as French, British and American troops pushed the Germans eastwards.
It became clear that the Germans and their allies could not win the war. As its troops retreated, Germany began to request an armistice with the Entente. Wilhelm's abdication paved the way for a ceasefire. On November 11, 1918 Germany established an armistice.
Therefore, with this the promised German victory had not been won. Germany's involvement in World War One had reduced the Entente's military resources on both fronts, but as fresh American troops supported Britain and France victory became increasingly less likely. The effectiveness of the Entente counter-attack in 1918 was a clear demonstration of their increasing military superiority, as Germany's armies retreated in the West. Germany accepted an armistice with the Entente, which was not entirely a negotiated peace, but ended the war.