During WWI (and immediately after) Germany was under British naval blockade. This meant not only that German people were under constant threat of starvation (some estimates put the number of deaths caused by starvation in the ballpark of 500000), but also that the whole war industry was having supply problems. Ersatz goods, like synthetic rubber and limited trade for valuable resources (via neighboring neutral countries or with blockade runners) can only get you so far. German troops would often describe abysmal condition of their equipment, with everything from uniform buttons and leather straps to artillery gun barrels being in short supply in addition to ever-present food shortages.

You also have to keep in mind, that even with Russia out of the war, Germany and Austria-Hungary was still fighting on multiple fronts, with Italian and Balkan theatre still open. With the entry of the US, with its fresh troops and untouched industry, the long term outlook was dire for the Central Powers. Basically, Germany was running out of everything that is needed to fight a war: manpower, money, food and industrial capacity.

German high command of course realized this, but they also knew that Entente situation wasn't much better at the moment. While supply problem wasn't as bad for Britain and France, both colonial powers with sizable navies and trade capabilities, manpower problem was at least just as bad. French army, with recent mutiny crisis was extremely demoralized, and Americans haven't arrived in sizable numbers yet, not to mention that while fresh, well fed and motivated, US troops were no match for the hardened veterans of the continental warfare, at least for the time being. Russia was tying up bulk of German resources in the East, so after Brest-Litovsk Germany found themselves with around 50 free divisions.

In March 1918 Germans started their big offensive, known simply as the Spring Offensive, Ludendorff Offensive or Kaiserschlacht. The idea was to gain as much as possible to force armistice, before newly arriving American troops could equalize the temporary disparity. No clear objective of the offensive was established, unlike with previous large scale battles of the western front, with the idea to exploit local tactical situation to gain ground and break through enemy forces. The ideal scenario was a wide breakthrough and outflanking the British forces holding the northern part of the front.

While the offensive itself employed number of revolutionary tactical innovations, such as widespread use of infiltration tactics by German stormtroopers and saw the biggest territorial gains in the West since 1914, it failed to break the French and British. By the April 1918 the danger of a wide breakthrough had passed and Germans used up their strongest troops, especially picked from every unit just to lead the charge in the offensive, enormous amount of materiel set aside for the offensive and were now in possession of vast amounts of relatively low value ground, which was hard to defend with depleted troops. What followed was the Hundred Days Offensive in the West, retaking all the ground lost and braking through the Hindenburg line, with new offensive in the Balkans from the Greek territory, which led Germany to sign the armistice in November 1918.

So long story short, Germany in 1918 was in no position to prolong the war, because of the blockade and the US entry into the war, took the opportunity that arose to finish the war before it became hopeless with the Spring Offensive, but failed win it decisively and force the French to surrender.