Complex sentence: Apart from Hitler, only General Friedrich Fromm, commander of the Reserve Army, could put Operation Valkyrie into effect, so he had to be either won over to the conspiracy or in some way neutralized if the plan were to succeed.
Split into simpler sentences: Apart from Hitler, only General Friedrich Fromm, commander of the Reserve Army, could put Operation Valkyrie into effect. Therefore, Fromm had to either be won over to the conspiracy, or in some way neutralized for the plan to succeed.