Final answer:
General Benedict Arnold during the Revolutionary War and Alexander Hamilton were not selected for military command, the latter causing political concerns. General Douglas MacArthur was relieved of command for insubordination, and General George McClellan was replaced for ineffectiveness.
Step-by-step explanation:
The individual who was upset for not being selected as the Commander of the army was General Benedict Arnold during the Revolutionary War. Arnold felt slighted when his contributions to the Battle of Saratoga were not acknowledged, which later contributed to his decision to commit treason. Another notable figure, Alexander Hamilton, was appointed as the de facto commander of the U.S. Army, which caused concern among many Republicans. Similarly, General Douglas MacArthur was relieved of his command during the Korean War by President Truman for insubordination, and General George McClellan was replaced by Lincoln due to his ineffective leadership.