In Germany, Nazi leader Adolf Hitler conducted a cold-blooded purge of his own political party by assassinating hundreds of Nazis whom he believed had the potential to become political enemies in the future. He ordered the members of his elite SS guards to murder the organization's leaders, including Ernst Röhm. Since then, the phrase ""the long knives"" has come to be an expression for acts of vengeance or any ruthless removal of unwanted associate.
The event is significant since it put Hitler at the peak of administration. It represented a big triumph for him and established him as the supreme leader of Germany, as he put it in his July 13 speech to the Reichstag.
Here is a video on the infamous Night of the Long Knives.