Complex sentence: Grant was one of the few senior general that Lincoln did not know personally, and the president was not able to visit the Western Theater of the war, but Lincoln came to appreciate the battlefield exploits of Grant despite complaints from others in the army.
Split into simpler sentences: Grant was one of the few senior general that Lincoln did not know personally, and the president was not able to visit the Western Theater of the war. But Lincoln came to appreciate the battlefield exploits of Grant despite complaints from others in the army.