I'm aware of the plots to assassinate him and Operation Valkyrie, but I'm wondering more about whether the beginning of the war was apprehensive for some, and if anyone tried to stop it.
The line in the sand for the German Generals was not the invasion of Poland. By then, it was already too late. It was the invasion of Czechoslovakia.
Their resistance to Hitler's war plans in 1938 and before the invasion of Czechoslovakia were the reason why he used the first chance to get rid of General Werner von Blomberg, then War Secretary and CiC of the Wehrmacht, as well as General Werner von Fritsch, then CiC of the Army, in the so-called Blomberg–Fritsch Affair: Hitler blackmailed Blomberg with the latter's much younger wife's past and had his henchman Heydrich disseminate rumours that Fritsch was homosexual. Von Blomberg was pressured into retirement, von Fritsch was transferred and later sought death in combat.
Even after Hitler forcibly retired 16 Generals and transferred 44 more, the head of the German General Staff Ludwig Beck tried to organize his peers against Hitler's war plans. A meeting that took place on August 4th 1938 showed that with the exceptions of Generals Ernst Busch and Walter von Reichenau, the entire General Staff presciently considered a war to be unwinnable and eventually to lead into catastrophe. In particular, General Erwin von Witzleben made plans together with, among others, Generals Franz Halder, Walter Graf von Brockdorff-Ahlefeldt, Paul von Hase and Erich Hoepner to arrest, imprison and eventually put Hitler before a court in the event of a Franco-British declaration of war. At the same time, Admiral Wilhelm Canaris, who was working with Beck and von Witzleben, sent Ewald von Kleist-Schmenzin to Britain as his envoy, asking for a British declaration of war in the event of a Nazi invasion of Czechoslovakia. That declaration would have given the General Staff both the pretext and support for the above-mentioned overthrow of Hitler.
Chamberlain's caving-in at Munich however destroyed the German generals' plans. Beck was eventually pressured into retirement, von Witzleben was transferred to Heeresgruppe two and Canaris shifted his efforts to more clandestine means. All of them were and eventually killed after the failed 20 July plot.
Other Source:
Joachim C. Fest: Plotting Hitler's Death: The German Resistance to Hitler, 1933-1945. Phoenix, 1996 - ISBN 0753800403