U.S. Defense Secretary Hagel Said to Step Down: NYT

The New York Times reports that Defense Secretary Chuck Hagel is resigning, possibly as soon as today. The President is expected to announce this in the Rose Garden. Hagel has faced increasing pressure due to national security concerns, particularly the advance of the Islamic State. His resignation is seen as a move to bring in a Defense Secretary with different skills. This marks the first cabinet-level resignation after the Democrats lost their Senate Majority in the midterm elections. Further updates will follow as more information becomes available.

What are the national security concerns mentioned in relation to Chuck Hagel's resignation?

Who is expected to replace Chuck Hagel as Defense Secretary?