Why (and how) did Germany renege on Brest-Litovsk only a few months after it was agreed to?
This is actually a followup to my "Did Germany really surrender in WWI" -- I have received excellent responses from Hi1954 and Ladyvols1, but unfortunately the website doesn't allow follow-up questions. Brest-Litovsk was mentioned in Ladyvols1's response and it baffles me to this day how Germany (who lost) was able to so easily not honor it a few months later.
After the new Russian government under Kerensky was overthrown by the Bolsheviks, the newer government of Lenin began to negotiate terms with Germany in January of 1918. The negotiation dragged on, Trotsky walked out in protest over the German's harsh terms, and the German response was to resume the war. This brought the Soviets to heel, and the treaty was signed in March.
German Foreign Secretary Richard von Kuhlmann decided to appeal to the left elements in Germany (and to undercut the Entente powers politically in the East) by appearing to renounce territorial claims the Reichstag wanted. He appealed to pro-German groups in the Baltic and the Ukraine, and under the guise of "self-determination" split them off from Russia. This would, he hoped, allow Germany to exploit the resources of these regions and thus regain strength Germany had lost through the effects of the British blockade. He also hoped this would work before the strength of the Americans on the Western Front and the weakness of Austria-Hungary forced a decision. The Soviets, in the midst of economic and political chaos, simply had no power to resist these moves. Lenin and his government had to concentrate on consolidating their control over the bureaucracy and raising an army to deal with the rising tide of civil war. The advance of German and Austrian troops into the Ukraine was made easier by the fact that the Turks did not take part in the Treaty of Brest-Litovsk, and continued fighting the Russians.
The Germans abetted their political efforts with troops freed up by the Treaty, no longer needed to fight on the Eastern Front. Other troops were sent to the West, where they took part in the Ludendorff Offensives. Unfortunately for Germany, the plan could never have worked. The German forces in the West were too weak to prevail any longer, Ludendorff picked the worst parts of the allied lines to attempt to force, and the tide of events had simply gone too far against them. American strength gathered too quickly, and General Pershing's refusal to allow his troops to be used piecemeal as replacements in French units by Foch meant that the power of the new troops could be brought to bear in coordinated fashion. Although the advances in the Ukraine went well, the collapse of Austria meant that these forces could not be brought back to defend Germany in a timely manner. In addition the grain and other supplies of the Ukraine could not be shipped to Germany in bulk quickly enough to alleviate the starvation of the people.