The period between the end of World War I in 1918 and the rise of the Third Reich in 1933 was a time of tremendous transition and great change for the German military. This period saw the end of the Imperial German armed forces, the fall the of the Kaiserreich, the establishment of the Weimar Republic, the rise of numerous ad-hoc, volunteer, paramilitary, counter-revolutionary Freikorps units, and the formation of 3 major versions of the official German armed forces culminating in the Reichswehr that would become the Wehrmacht in 1935.
Germany signed an armistice ending World War I on November 11th, 1918. After the armistice was signed the Imperial German armed forces withdrew from their various fronts and began returning to a defeated Germany en masse. It didn't take long before returning German units began to dissolve as the toll of war on soldiers and civilians alike pulled apart any remaining shroud of discipline that once kept them intact.
With the end of World War I also came a sweeping wave of revolution and counter-revolution across most all of Western and Eastern Europe. Germany was not immune to the fervor of revolution and experienced numerous uprisings, coup attempts, counter-revolutionary assaults, street battles, and territorial land grabs. The front soliders that largely disbanded and dissolved upon returning home to Germany fought on both sides of these revolutionary struggles.
A great number of former WWI soldiers went on to form or join ad-hoc volunteer units collectively known as Freikorps. Freikorps units could consist of small groups of less than 100 men loosely thrown together along quasi-military lines to defender local areas, while others were divisional sized formations consisting of infantry, artillery, machine gun and motorized units, logistical support, engineers, and air power. Estimates put the number of Freikorps units formed during the period 1918-1923 at about 200-300. Freikorp units served as the basis for combating communist revolution across Germany, saw service in the Baltic region and fought the Poles along the eastern frontier defending against various Polish territorial incursions.
Since most vestiges of the Imperial German army had all but disappeared shortly after the end of WWI a formal military force was needed by the new Weimar Republic, aside from the largely independent and unregulated Freikorp. on March 6th, 1919 a new German armed forces was formed through official German decree known as the Vorläufige Reichswehr, or Provisional German Defense Force. The Provisional German Defense Force consisted of the Vorlufige Reichsheer and the Vorlufige Reichsmarine. Many Freikorp units served partly or entirely as the basis of the Vorlufige Reichsheer which consisted of about 400,000 men in nearly 50 Brigade sized units.
On September 30th, 1919, all ties with the Imperial era were broken when a totally new army was formed, now known as the Übergangsheer, or Transitional Army. Again, Freikorp units served partly as its basis which consisted of 30 Brigade sized units. The Vorlufige Reichsmarine continued to be known as such until 1921.
On June 28th, 1919, Germany signed the formal Peace Treaty sealing the previous armistice of November 11th - this was the well known Treaty of Versailles. According to the treaty Germany was allowed a standing armed forces of 100,000 men. This new and highly regulated force was to be known simply as the Reichswehr which was officially formed on January 1st, 1921. It consisted of the newly named Reichsmarine and Reichsheer. The Reichsheer consisted of 2 Group Commands, 7 Infantry Divisions and 3 Cavalry Divisions. The Reichswehr and Reichsmarine would exist until 1935 when the WWII-era Wehrmacht was formed..