In US, President-elect Donald Trump has picked retired Marine General James Mattis as his Defence Secretary, but Mattis will need a change in a federal law to be able to take the job. <br/><br/>Trump, appearing at a post-election victory rally in Cincinnati, Ohio, made his choice official after a day of unconfirmed media reports. But for Mattis to be approved by the US Senate, Congress will have to waive a law requiring Defence Secretaries to be retired from active service for at least seven years. Mattis retired in 2013.