Secretary of State Rex Tillerson entered office with grand ambitions, but he'll leave as America's most isolated and unpopular chief diplomat for many years.

Now likely to be replaced by CIA Director Mike Pompeo, Tillerson's short tenure in Washington will be remembered for its one sustaining lesson: To survive in government, you need an island. Put another way, you need allies.

Back in February, Tillerson seemed set for success. "I know nobody will always be perfect, and that certainly includes me," the secretary told State Department employees on his first day in office, "But I ask that everyone strive for excellence and assume responsibility for their actions and their decisions."

Skeptical but patriotic, American diplomats were originally optimistic about Tillerson's reformist agenda. The secretary's plan to slash a top-heavy bureaucracy, improve its decrepit technology systems, and empower young diplomats abroad seemed exciting. These proposals might even have given Tillerson the employee buy-in to eliminate wasteful programs and give taxpayers better value for money.

But first, Tillerson had to earn the trust of his personnel.

And he never tried.

Instead, the secretary surrounded himself with a tiny group of confidantes. For top career officials, even getting a meeting with Tillerson has been a coup. For mid-level officers, Tillerson's total disinterest in filling vacant political appointee positions has engendered confusion and dismay. For junior officials, Tillerson's relentless focus on spending cuts has fostered doubts about once-exciting futures.

Quickly and irreversibly, Tillerson lost his department.

In turn, Tillerson became that much more vulnerable inside the White House.

When it came to winning Trump's favor, Tillerson was always going to struggle. Lacking the combat record and military disposition of Secretary of Defense Jim Mattis, Tillerson lacked the means of earning Trump's reflexive trust.

Yet the secretary's greatest mistake was in attempting to conduct foreign policy outside of Trump's direct control. It didn't go well.

As White House officials increasingly insulted him in the press, Tillerson pushed back hard, apparently describing his boss as a "moron."

Bad move.

By August, the president was openly contradicting his second most senior Cabinet official. Tha signaled the end for Tillerson at home and abroad. After all, if foreign governments cannot trust Tillerson to speak for the president, he isn't worth listening to.

By October, the situation became embarrassing when Trump escalated to criticizing Tillerson on Twitter.

By November, things became dangerous as Tillerson insulted close U.S. allies in a misguided effort to annoy the president.

And so we are where we are now. Tillerson has managed to unify Trump and State Department employees in one shared opinion: Rex must go.

Alone at the heart of government, Tillerson holds tight to his title and his pride. Soon he'll have only private sector luxury and Wikipedia posterity.

Mr. Pompeo should be wary. Whatever else he does at the State Department, he must first make Foggy Bottom his island.