As Yasskier stated, Ernst Röhm and his Sturmabteilung, SA, or Brownshirts began to be perceived as a threat to Hitler's leadership. They had been the original paramilitary wing of the Nazi Party. Their membership was made up of working class blue collar men, who were interested in the socialist component within the National Socialist Party. This sometimes put them at odds with Hitler. This also made them an independent power base inside the Nazi Party. A power base which was not always in locked step with Hitler.
Ernst Röhm controlled the Brownshirts; as the SA's popularity and prestige in the Nazi party grew, he came to be seen as a rival to Hitler. Hitler's inner circle, including Heinrich Himmler (who led the SS) and Hermann Göring (who had founded the Gestapo) sought to purge the SA from the Nazi Party so they could take over their portfolio and build their own power bases in the vacuum. At first Hitler resisted them. Then it became politically expedient for Hitler, in order to form a political alliance with the German Army, to utilize Himmler and Goring to purge the SA from the Party. This political alliance with the Army would not only bring Hitler to power in the German government but through purging of the SA, would allow Hitler to re-organize and consolidate the Nazi Party underneath him. After this, Hitler would become the undisputed leader of both (the Nazi's and the Government), and any opposition to his policies had a powerful demonstration of how the Nazis took care of those who opposed them.
Ernst Röhm's undoing was his ambition. He had dreams of his Brownshirts becoming the new German military. The "people's army", replacing the Reichswehr, the Weimar Republic's army. By the time Röhm was murdered in 1934 the SA had over 3 million men, dwarfing Germany's professional army which was limited by the Treaty of Versailles to 100,000 men. The army, which could trace their service back to Fredrick the Great, resented this. They believed the SA little more than thugs, and had no desire to serve under their leadership.
As the German President Paul Von Hindenburg's health declined, Hitler required the support of Germany's armed forces in his quest for political power. The Army's price for supporting Hitler's quest for power was the suppression of the SA, which they saw as a threat to German's military tradition and their leadership of Germany's armed forces. This ultimately resulted in the Night of the Long Knives, also known as the "Röhm Purge", where the SA leadership was brutally and violently purged from the Nazi Party. This also lead to the execution of Röhm himself; and Hitler coming to political power within Germany.
On 11 April 1934, Hitler met with German military leaders on the ship Deutschland. By that time, he knew President Paul von Hindenburg would likely die before the end of the year. Hitler informed the army hierarchy of Hindenburg's declining health and proposed that the Reichswehr support him as Hindenburg's successor. In exchange, he offered to reduce the SA, suppress Röhm's ambitions, and guarantee the Reichswehr would be Germany's only military force. According to war correspondent William L. Shirer, Hitler also promised to expand the army and navy.
Defence minister Werner von Blomberg issued an ultimatum to Hitler from Hindenburg: unless Hitler took immediate steps to end the growing tension in Germany, Hindenburg would declare martial law and turn over control of the country to the army. Knowing such a step could forever deprive him of power, Hitler decided to carry out his pact with the Reichswehr to suppress the SA.
So Hitler did not get rid of the SA which the Reichswehr despised, so what changed to the SA afterwards that it became accepted to the Reichswehr? –
I don't think the Reichswehr ever "accepted" the SA. The SA ceased to be discussed as an army and was utilized for duties the Reichswehr saw as beneath them. The SA were enlisted for vandalism, arson, and general extrajudicial violent suppression of domestic Germany, primarily Jews.
Initially Hitler decapitated the SA. Using the Schutzstaffel (SS) under Himmler and the newly formed Gestapo under Göring to hunt down and murder the leadership of the SA. Estimates on how many people were killed range from a low of 85 to over 1000. Additionally, more than 1000 others were arrested. The purge wasn't entirely SA members, as any group which expressed independence from Hitler within the Nazi party was a target. Mostly though it was directed at the SA leadership.
The Night of the Long Knives was an important event in Hitlers expressing complete control of the Nazi Party and control of the German government. It was the event which made Hitler the undisputed leader and sole voice for both.
After the Purge, Hitler proclaimed the murders and arrests had taken place because Röhm and the SA had conspired to seize power. Hitler appointed new leadership (Viktor Lutze) to the now out of favor SA, and it's membership plummeted. Within a year of the purge their membership was down to 40% of it's pre-purge size. The new smaller organization was used in the coming years as the primary organization to attack Germany's Jewish population. They played a key role in Kristallnacht (Crystal Night) where thousands of Jewish stores, homes and cemeteries were vandalized, most of Germany's synagogues were burned, and 30,000 Jewish men were rounded up and taken to concentration camps.
How big was the SS at that point? You wrote that the SA was under the command of Viktor Lutze after the purge, but when was it under Himmler's command? I thought Himmler was the head of SS + SA?
Himmler never commanded the SA. He commanded the SS and even the Gestopo after Goring. The SA continued on as a lesser organization. They lost most of their membership to service in the army. Late in the war their was an attempt create a military branch similar to the Waffen-SS; the Feldherrnhalle SA-Panzergrenadier Division, 1944-1945. They eventually surrendered to the US Army, who turned them over to the Soviet Red Army. – J
@Zebrafish and Diger.. January 30, 1933, SS membership had increased to over 50,000; up from 300 when Himmler took command of them in Jan 6, 1929. How were they so effective against the much larger SA? Organization, surprise, familiarity with the SA, superior discipline and ruthlessness; all allowed the SS to pick out the key personnel in the SA to make a difference. Himmler didn't attack 3 million SA members, He attacked just a few thousand higher ups( up to 1000 murdered, thousands more arrested). That and Hitler's denouncement paralyzed the SA, which wasn't really known for their own organizational skills but their thuggish street brawling behavior. Ultimately Hitler used the SA's own command structure against them. He appointed new leaders for the ones he dispatched and just assumed control of the entire organization. Then they managed them into obscurity.