In the days preceding England’s final ultimatum to Hitler on September 1, 1939, following his attack on Poland, Downing Street received intelligence reports indicating that there was dissension within the German general staff. The opposition to Hitler was said to be loosely based around the German army chief of staff, Ludwig Beck, who was to pay with his life five years later in the aftermath of the failed July plot to assassinate Hitler.
These reports were true. There was no conspiracy among many senior staff, nor any united action, but, among many, a feeling of malaise, of doubt, and an acute lack of confidence in Hitler’s competence. Major-General Georg Thomas, the head of War Economy and Armaments at Supreme Headquarters, drafted a memorandum to Hitler that contrasted the German economy with that of the Western Powers including the United States. Efforts were made to impress upon General Fritz Halder, the army chief of staff, and upon Walther von Brauchitsch, the army commander-in-chief, the urgent need to obstruct Hitler’s drift to war. But both senior commanders refused to see the doubters.
One of them, the counter-intelligence officer Hans Oster, interpreted Hitler’s decision of August 26 to delay the order to attack Poland as a victory for peace. “The Führer is done for,” Oster said, according to a bystander who reported this later. “It is now merely a question of time and manner how this unmasked imposter could be removed with the least trouble and the most elegance.”
Source: Richard Overy, Countdown to War, Penguin Books, 2009.
Eric Koch’s new book, The Weimar Triangle, is available at Indigo-Chapters and in your local bookstore. 
In his Fateful Choices; Ten Decisions that Changed the World 1940-1941, Ian Kershaw puts it this way:
“Moreover, Hitler could always reckon on high levels of support within the ruling elites, most importantly in the leadership of the armed forces. When, in the summer of 1938 General Ludwick Beck, the chief of the General Staff, fundamentally opposed Hitler’s decision to attack Czechoslovakia that autumn (later postponed because of the western powers’ intervention at the Munich Confrence) he found himself completely isolated within the army leadership and his resignation from office in despair had no effect whatsoever on poicy.”
True.
When in December 1941 the Germans failed to take Moscow, certain generals told H. the war was no longer winnable.
They were overruled. Either before or after that, H. declared war on the U.S.
It is actually amazing that there were not more attempts on Hitler’s life. In today’s world it is unimaginable that the Prussian military would wait until July 1944 to try it and then botch the attempt so badly. The only high-ranking Nazi leader assassinated during the war was Reinhard Heydrich who was killed in Prague in May 1942 by Czechoslovak parachutists sent from England. The Nazis took terrible revenge.