Chuck Hagel has resigned as defence secretary in a major shake-up to a US national security team that has been beset by crises around the world.
President Barack Obama has faced considerable pressure to reshuffle his senior national security officials against a backdrop of widespread criticism that the administration has fumbled its response to crises from Ukraine to Iraq.
The only Republican in Mr Obama’s cabinet, Mr Hagel is the first high-level official to leave the administration after the Democrats were roundly defeated in midterm elections earlier this month and lost control of the Senate.
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