In his resignation letter, Defense Secretary James Mattis appeared to point to his policy differences with President Donald Trump, who has frequently clashed with U.S. allies.

Write a formal resignation letter once you have talked to your supervisor. Resignation letters do not have to be long. But, use this opportunity to express and recount all the positive opportunities ...

In an abrupt departure, Homeland Security Security Secretary Kirstjen Nielsen resigned from her post on Sunday. Read her the full letter.

1 Write a hard-copy resignation letter Don't send it electronically; e-mails can be too easily rerouted and circulated. Use a blank, letterhead-free piece of paper and sign it at the bottom.