It was confirmed by the Pentagon on Monday morning that U.S. Secretary of Defense Chuck Hagel is stepping down from his post. A senior official at the defense department says it was a "mutual decision" between Hagel and the White House after weeks of discussions about the direction the administration wants to go in its final two years.

While Hagel's time in the position may seem short, historically it is not unprecedented. In fact, most secretaries of defense have not stayed on for a full presidential term -- a majority not lasting 3 years for one reason or another.

There have been 24 defense secretaries in United States history. The man who spent the shortest time in the position was Elliot Richardson, who served 114 days under Richard Nixon in 1973 -- 4 months before being appointed attorney general. The longest serving was Robert McNamara under John F. Kennedy and Lyndon B. Johnson. He served a total of 2,595 days -- almost two full presidential terms.

Reports are conflicting on what exactly led to Hagel's departure and while a defense official says it was a mutual decision, several news outlets are reporting that an administrative official says it was not as amicable as some are suggesting and that Hagel was forced out after months of not being fully integrated into Obama's inner circle and the decision-making process.

In an announcement on Monday, President Obama thanked Hagel for his service and Hagel gave a professional and gracious farewell -- what one might expect from these events. However, no reason was given as to why Hagel was departing as the timing seems odd with everything going on in the Middle East -- not even talk of a mutual decision.

Politicos will continue to speculate on all the factors that played a role in Hagel's departure and who the president will choose to replace him. Hagel said he will stay on until a successor is confirmed.