Complex sentence: Devers did not have much faith in either of them, even before Lucas' relief; but Marshall insisted that an experienced commander was required, and with Patton gone, Clark was the only one in the theater.
Split into simpler sentences: Devers did not have much faith in either of them, even before Lucas' relief. But Marshall insisted that an experienced commander was required, and with Patton gone, Clark was the only one in the theater.