Oh there's so many that I want to talk about. The French Marshalate was full of petty rivalries that helped lead to the destruction of Napoleonic in France. I know Lannes and Bessiers hated each other but I'll highlight the really juicy ones.

In 1807, the double battle of Jena-Auerstedt is the absolute high point of French military skill, the best of the best were on the battlefield. However, the best was also divided. Between Jena and Auerstedt, Marshal Bernadotte was waiting. Both Napoleon and Davout were calling on him to help them, but Davout needed it more because he faced the actual Prussian army almost three times his size. A combination of Prussian ineptitude and Davouts genius allowed for a victory he shouldn't have been given. However, from that point Davout hated Bernadotte, he wanted revenge for Auerstedt. It got to a point that in 1813, Davout demanded to fight the newly crowned Prince of Sweden, Charles XIV (Bernadotte).

Or even better, the best is the rivalry of Ney and Massena. Ney didn't like being under anyone but the Emperor, so he disobeyed orders of Massena who was in command of all French forces in Spain. In an effort to push the British out of Iberia, Massena pushed into Portugal but failed. In the retreat, he ordered Ney to command the rearguard because he thought it would be a convent way to dispose of an unruly subordinate. The opposite happened; in what can only be described as a prelude to the Retreat from Russia, Ney conducted a glorious rearguard action that surprised the British and kept the French army safe. In an effort to get Ney killed, Massena only showed that Ney had much to give to France.

Oddly enough, the Napoleonic age was full if rivalries.

The most prominent rivalry in the 5th century Mediterranean was between Athens and Sparta, and more specifically, the Delian and Peloponnesian Leagues, respectively. To understand their mutual distrust and rivalry, back-story is necessary, which will be addressed here:

Before the Persians (Darius and then his son, Xerxes) attempted their conquest of Hellas, the Poleis (city-states) of what we now know as Greece were constantly at each other's throats, fighting small-to- medium sized wars that didn't last very long, and that were usually resolved in one battle in a pre-arranged place. City states that were prominent on the battle stage before the Persian wars were Sparta, Corinth, Athens, Thebes, and Megara, to name a few.

The failed invasion of Darius in 499 B.C. that led to an embarrassing defeat at the hands of united Greeks at Marathon, followed by a slightly more successful conquest in 480B.C. by his son, Xerxes, which was thwarted at the battle of Salamis united Greece in a way never seen before. Before all this, Sparta was understood to be the uncontested superior militaristic force on land, and very few city-states wished to meet a Spartan force on the battlefield. Thusly, many city-states near Sparta made treaties with Sparta which culminated in the Peloponnesian League in the 7th century, a defensive alliance. After the Persians were defeated once- and-for-all, the citizens of Hellas started to respect Athenian naval power, which had stood the test of the Persians, the greatest force at the time. Thusly, the Athenians, with their new-founded power, created the Delian League after the Persian wars.

So this split Greece in half, some city-states in favour of Sparta, and others in favour of Athens. For various reasons involving these two powerful and well-equipped Leagues, their alliance and land interests began to overlap, which culminated in the infamous "Peloponnesian War" which utterly destroyed Greece from 431B.C. to about 403B.C. It pitted the strong and phalanx-friendly Spartans, with the savvy and naval Athenians: a true "Elephant vs. Whale" situation.

The wars include massive victories and shameful losses from both parties and "total war" - it left Greece vulnerable to the Macedonians later in the 4th century (King Phillip and ultimately, Alexander the Great) to take it over.

source: "The Peloponnesian War", Donald Kagan, 2003, Penguin Books.

Carthage and the Republic of Rome. They were friendly for hundreds of years, though nothing more than signing a treaty realizing their perspective areas of influence in 509 BC, and then later again in 348, 306 and 279. Carthage even offered Rome assistance in their war against Pyrrhus, which they declined.

Then, the Punic Wars happened. Three bloody struggles with some of the most famous commanders of either side, as well as history, shining in these conflicts. Names like Hannibal Barca, Scipio Africanus, Fabius Maximus. The First Punic War was fought predominantly around Sicily and surrounding islands, and lasted 23 years, and included Rome's rough rise as a naval power. The Romans suffered so badly from storms, disease and war that their population actually dropped slightly. In the aftermath, Carthage faced the most serious threat it would face until the 3rd Punic War when it's mercenary army rebelled against it following a payment dispute.

The 2nd Punic War was where Hannibal comes to shine. He marches to Italy via the Alps, and spent 15 years in Italy, the pinnacle of his time there being the famous Battle of Cannae, which is still studies in military academies to this day.

I just realized the time and I need sleep. I'll see if I can continue this tomorrow as I am passionate about Carthage and the Phoenicians, but I really, really need sleep.

I'm going to go with John Adams and Thomas Jefferson. Their relationship is fascinating and should be part of American lore more so than it is now. Politically they were fierce rivals, Jefferson was the de-factor representative, leader and embodiment of the Democratic-Republican party. Adams was the first Federalist president (if you count Washington as non-partisan, which I do) and he and Jefferson constantly argued over the merits and dangers of strong central government. Adams was a major proponent of Jefferson and was the main backer behind having Jefferson write the monumental Declaration of Independence (along with assistance from Benjamin Franklin). There is always an evident admiration, at least of ability and aptitude, between the two. They both served in Washington's cabinet, Adams as Vice President and Jefferson as Secretary of State. However frequent conflict between the two lead to resignation by Jefferson from his office (something Washington never quite forgave him for). In the extremely tight election following Washington's presidency, Adams was named the president but because of how the electoral process worked at the time Jefferson was Adams' Vice President. This presented an extremely inefficient executive process because despite Jefferson and Adams being two of the most brilliant men of time, their ideologies were conflicted. In their rematch, Jefferson would claim victory and Adams became the first president to serve only one term in office. Here is a link of some of their correspondence after both had left office. http://nationalhumanitiescenter.org/pds/livingrev/religion/text3/adamsjeffersoncor.pdf . Finally in almost scripted fashion, Adams and Jefferson both passed on the same day within hours of each other, on the fiftieth anniversary of the signing of the Declaration. (Note that some claim that Adams' last words were "Jefferson lives." However there is no concrete evidence that this isn't a tale or folklore that evolved from the already highly coincidental passing of both men).