Complex sentence: Upon his arrival in France in June 1917, American Expeditionary Force commanding general John J. Pershing met with Mitchell, who advised Pershing that his office was ready to proceed with any project Pershing might require.
Split into simpler sentences: Upon his arrival in France in June 1917, American Expeditionary Force commanding general John J. Pershing met with Mitchell. He advised Pershing that it was ready to proceed with any project Pershing might require.