Complex sentence: He had doubts about the information, which he noted in his private diary but did not express openly, but Adolf Hitler was convinced of the veracity of the bogus documents, which reinforced his own concerns for the region.
Split into simpler sentences: He had doubts about the information, which he noted in his private diary, but didn't express openly. But Adolf Hitler was convinced of the veracity of the bogus documents, which reinforced his own concerns for the region.