... plotted to assassinate him.
This was the famous "July Plot" of 1944, a plot by senior military officials to detonate a bomb that would kill Hitler. The briefcase bomb exploded, but Hitler himself sustained only minor injuries. (There were some other persons killed by the blast.) Those behind the plot believed Hitler was leading the nation to a catastrophic conclusion to the war, and wanted to replace his government in order to work out a manageable treaty with the Allies who fought against Germany.
Hitler's response was more of his unstable and maniacal approach. He had hundreds of people arrested on suspicion of conspiracy with the plot, and about 200 were tried and executed. Some of these executions by slow strangulation with piano wire were filmed for Hitler to be able to watch.
"Operation Valkyrie" was a name given to part of the plans against Hitler, and that name has become associated with the assassination plot overall. A film dramatization, called Valkyrie, was made about this in 2008.