Diplomacy is meant to be more difficult to practice on hostile terrain than in the capital of a key ally. The British Ambassador in Tehran or Moscow must acquire information in creative or covert ways.
Among friends, information is meant to be shared.
That is why the job of British Ambassador in Washington is regarded as the finest and most rewarding diplomatic posting in the world. Yes, your Residence is a magnificent 1920s mansion designed by Sir Edwin Lutyens.
But it is not the architecture of your home that is the appeal of this particular ambassadorship.
Rather, it is something else, something a little bit more magical, elusive, rare, seductive.
In a word: Access.
The British Ambassador here is seen as a player on the big stage. Someone consulted by the White House, the State Department, the National Security Council, Congress, the intelligence agencies.
Indeed, it is sometimes even more than that. Occasionally, warring Republicans and Democrats will sit in the Residence and - helped by some British hospitality and a little whiskey - actually do business together.
The British Ambassador's home has been described as a safe space in a tribal town.
But then came the Trump earthquake.
The new American President had no interest in the diplomatic corps. He saw Washington's foreign envoys as innately pro-Democrat Establishment figures. In this regard, Trump was probably right.
Even Sir Kim Darroch struggled mightily to make friends in this Administration. A few White House advisers would attend Embassy parties but it was always awkward to watch - like observing two species interact for the first time.
Suddenly, instead of enjoying the access of his predecessors, Sir Kim was being ignored.
When Trump made the impulsive decision to withdraw US ground troops from Syria, the British weren't even informed, even though we too had soldiers there.
When Trump abandoned the military strike on Iran at the last minute, Sir Kim was in the dark. He wasn't briefed about the attack, nor of the change of course.
So the allure of the job here had already diminished. But the real difficulty came with the leak of the diplomatic cables on Sunday. Trump instantly moved from indifference to hostility.
He now saw Sir Kim as foe, not friend.
And what is the ultimate humiliation in this power-crazy town? Denial of access. So the White House reversed his invitation to a dinner, and hinted he would not be allowed into the White House.
That made his job impossible and his position untenable.
The bigger question is who succeeds Darroch and whether the new ambassador will be able to regain the trust of the White House.
I wouldn't bet on it. President Trump won a victory this week. He forced out a high-profile ambassador and sent a message to all foreign missions in Washington: The old rules don't count anymore; Access to the Administration will be traded like a commodity; we want supplicants, not equal partners.
It is America First, certainly. It is also Diplomacy Last.