Nic Robertson is CNN's international diplomatic editor. The opinions in this article belong to the author.

(CNN)

In his first few months as US Secretary of State, Rex Tillerson was an enigma. He barley spoke publicly -- and when he did, in private, he rubbed his fellow diplomats the wrong way.

I first saw him in action soon after he took office at Nato HQ in Brussels at the end of March last year . He'd arrived as Donald Trump's bagman. His job was simple: deliver a tough message, telling NATO member states that they needed to pay up. Then he rode out of town as fast as he rode in.

He was rough around the edges, smart and bullish. He was almost a caricature of what his international diplomatic counterparts expected of a oil executive from Texas. He was used to making decisions for himself and leading from the front. And that was his downfall.

When he got the call from Trump to put a hold on his richly earned retirement from years at the helm of Exxon, his wife told him it was a calling from God to serve his country and he should not turn it down.

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