Chuck Hagel, Vietnam war veteran and former Republican senator resigned from the post of US defence secretary on 24 th November 2014. The resignation was due to the pressure on his strategy of fightings against Islamic state (IS) in Iraq & Syria. But he would continue his post till a successor is found.
Hagel was appointed as defence secretary with strategy of bringing back troops from Iraq & Afghanistan thereby reducing the defence budget. But in due course all changed with Islamic state uprisings in Syria & Iraq. The US ordered strikes on Islamic state strongholds to stop their advancements.
After the US midterm elections in November 2014, the republicans took majority control over the democrats, so Barrack Obama, the democratic party President & his team were under pressure over national security issues.
Because of this issues Hagel was pressurised to resign over his more war campaigns.