Was it the al-Assad conflict that caused Hagel to resign?

The news about the resignation of U.S. Secretary of Defense Chuck Hagel came right after it was announced that the nuclear talks with Iran had been extended to the end of June 2015.

There have been a number of speculations in the American media about President Barack Obama’s possible moves after failing in the mid-term elections, including changing Secretary of State John Kerry and National Defense Advisor Susan Rice. But Hagel’s name was not mentioned very much on those lists of educated guesses in the DC beltway.

Speculation started immediately about the reasons for Hagel's resignation. Some centered on him being the only Republican in the Obama administration and others mentioned the discrepancies between Hagel’s performance in Afghanistan and the strategy in the fight against the Islamic State of Iraq and the Levant (ISIL). But the one that seems most probable is the one with links to Turkey and Iran: Hagel's conflict with Obama over whether to include the removal of Bashar al-Assad in Syria in the target list in the fight against ISIL.

It seems that the story had started to surface a little before the mid-term elections on Nov. 4, with a New York Times report on Oct. 30 about a two-page memorandum by Hagel to Rice suggesting that the broader Syria policy should be part of the fight against ISIL.

That was actually a softer version of something that has been highlighted by the Turkish government for months. President Recep Tayyip Erdoğan and Prime Minister Ahmet Davutoğlu have been saying ever since the ISIL forces seized Mosul and attacked the Kurdish-populated Syrian town of Kobane, (near the Turkish border), that only if the U.S. commited itself to the removal of al-Assad as a part of the campaign against ISIL,...