4 epic rivalries between military commanders once on the same side

Throughout military history, it was common for military commanders to only know of each other by reputation or by the deeds of their troops.

But when lines are drawn, ideologies change, and another war is fought for another reason, you may find yourself fighting against your former allies and those old interpersonal rivalries can get ugly fast. It takes a darker turn when both sides of that rivalry have an army ready to kill and die at their military commanders word.

One man would later be known as the "Father of America" while the other would become synonymous "traitor." Both Washington and Arnold were heroes of the American Revolution early on and fought many battles together.

This was until Arnold switched allegiances back to the crown. His reasons for turning his back on America are still debated by historians, but the accepted reasons include money, disillusionment, and personal vendettas against the Continental Congress.

Both Generals would earn historical prestige leading their respective armies against each other during the American Civil War, but they weren't always enemies. In fact, at the beginning of the Civil War, Lee was offered command of the U.S. Army before resigning his commission. Eight days later, he accepted command of Confederate troops in Virginia.

Back in the Mexican-American War, however, both men fought side-by-side as then-Lieutenant Colonel Lee led troops in Scott's March on Mexico City with a young then-First Lieutenant Grant. Both Lee and Grant marched under the command of then-General Zachary Taylor. In fact, the Siege of Veracruz was full of names that would eventually become essential pieces of the Civil War, including future Generals Meade, "Stonewall" Jackson, and Longstreet.

Petain rose in rank to eventually become Marshal of France and, later, Prime Minister of the Nazi puppet state, Vichy France. He took strong and direct opposition to Charles de Gaulle's revolutionary Free France. After the fall of the Nazi Regime, Petain was spared the gallows because of his actions as "The Lion of Verdun" and hero of France. France's new leader, Charles de Gaulle, refused to execute the disgraced former-Marshal.

Petain's military mind helped save France in WWI at the Battle of Verdun. One of the most heroic battles and early turning point of Verdun took place when the Germans were contained at Douaumont and surrounded by 90,000 men and 21,000 tons of ammunition. There stood de Gaulle, the then-Captain in the French 33rd Infantry Regiment, leading Petain's charge. Charles de Gaulle was wounded and captured in that battle.

While Mao Zedong is etched in history as the Chairman of the People's Republic of China, his main rival was Chiang Kai-shek, the Chairman of the National Government of China (or, as it's more commonly known in America, Taiwan). To briefly summarize a long, storied conflict, both of these nations claim to be China. As the Communist Revolution swept over the mainland during the Chinese Civil War, the capitalists fled to Formosa (the island of Taiwan), but neither ceded statehood.

Just like the nations they led, Mao and Chiang have a history that oscillates between cooperation and opposition. First, they supported each other during the Northern Expedition. Then, they went at each other's throats during the Chinese Civil War. Then, during the Second Sino-Japanese War, when Axis-aligned Japan invaded a Soviet- and American-backed China, they allied again.