Prime Ministers usually ride high at least once. So it was on Friday night, when Theresa May (above) looked in general to have pulled off a tricky mission on her trip to Washington, DC. The new US President, Donald Trump, is deeply, deeply disliked in Europe: Mrs May had to get close enough to him to keep dreams of a decent trade deal alive (and the Western security alliance on the road), while not alienating her own voters. When she walked out of the White House - and misguided, unfortunate handholding aside - she looked like she'd managed it. Taken together with a forceful speech on the likely speech of Britain's exit from the European Union, and a well-received address to US Republicans that even contained some shade and nuance, most observers were thinking: is it skill, or just luck, that keeps her bandwagon on the road? Whatever it was or is, everyone could do with some.
But that was Friday. By Saturday night, the visit looked like a burning car, overturned in the rear-view mirror as everyone tried to get out the neighbourhood. News of President Trump's ill-conceived and deeply counter-productive asylum and immigration Executive Orders had broken, to widespread protests across both the US and UK. That was bad enough. But Mrs May's refusal to come out against them during her visit to Turkey made the impression that she was hand-in-glove with a dangerous right-wing demagogue stick to her like oil from the aforementioned car wreck. Basically, diplomatic victory had turned to political defeat in not much over 24 hours.
Now Mrs May will get away with this one, if she learns her lesson: not to get quite so close, physically, politically, imaginatively - to The Donald. She's going to have to get pretty close, now that Britain's going for a complete and clean break with the EU, but cosying up itself has got to be off the agenda. Otherwise, she's going to get burned by every single stupid stunt, nasty mendacity and straight-out, burn-the-world nihilism he comes out with during the next three years and ten months.
Mrs May doesn't need to worry too much at the moment. The main Opposition Labour Party isn't up to a pillow fight right now. If there was any real action, they'd disappear in a cloud of their own making. They don't even look like they want any power, let alone think that they should have power. But one day there'll be an opposition worth the name, and she will have to concern herself with what the voters think. In the meantime, there's that more amorphous beast to deal with: Conservative MPs, the views of party members and the deep, slow-to-change feelings of Middle England that lie behind them. If she allows the impression to gain a hold that she is leaden-footed, slow to react, not decisive in a crisis, it's feeling there will slip remorselessly away from her. Then she'll really be in trouble.
None of this comes as much of a surprise to historians. Prime Ministers have always walked a tightrope between success and failure. The gap between them is often much thinner - and the time taken to descend from one to the other much shorter - than you would think. Anthony Eden, elected in a landslide victory of his own in 1955? Humiliated by the catastrophic Suez Crisis in 1956, and bundled out of office, shunted onto a good long 'holiday', early in 1957. Harold Macmillan, again triumphant at the polls in 1959? By 1963, he was so distressed and wrung out personally, and his stock has plummeted so quickly amidst recession and a failure to get into the European Economic Community, that he announced his resignation to a stunned Cabinet that would happily have soldiered on with him as its leader.
You could play this game with all of them. Harold Wilson, swooping down decisively on the crisis when the Torrey Canyon oil tanker smashed into the Isles of Scilly off Cornwall in the spring of 1967, never recovered from his devaluation of sterling the following November. After three years defending sterling's parity, when he tried to make a virtue of its new dollar value, it just did not ring true: nothing was ever quite the same again. Edward Heath, triumphant in finally getting Britain into the EEC, brought low by the oil crisis in February 1974; Jim Callaghan, who skilfully brought country and party through the IMF loan and pegged inflation back, dragged down by the Winter of Discontent; it gets to them all in the end. Margaret Thatcher thought she could buck that tide, of course, but her own intransigence over Europe and the Poll Tax did for her; even after John Major won the unlikeliest of come-from-behind victories in the General Election of 1992, he faced his own devaluation catastrophe just five months later. Tony Blair's Iraq, and Gordon Brown's election-that-never-was, well, you know about them.
David Cameron? Well, when he stood on that table cheering on the Liberal Democrats' drubbing in May 2015, he thought he had finally secured a Parliamentary majority. Little did he know that he had replaced one set of herbivorous and harmless housemates with much, much more dangerous enemies: extreme Eurosceptic nationalists who were determined to bring him down at any cost, a mission in which they proved successful.
Mrs May should take heed: one day, one of these sentences will apply to her. Something out there - maybe Trumpism, maybe Brexit if it turns out to be an economic disaster, more likely something completely different - will bring her down. The seeds of that fall from the sun will probably be planted at a moment when she feels most successful - closest to the heat and warmth of power itself, in fact. A bit like she probably did last Friday.
Are there lessons from all this? Well, beyond the injunction to get done what you want to do, as fast as you possibly can, while you have power, there might not be that many elements that can be anything like codified. One important point is that you will probably make your most important mistake at the moment of your greatest success, partly because of overconfidence, partly because of events pressing around you, possibly because no-one will say 'no' to you. Watch for that. But probably the deepest lesson is this: your days in No. 10 are numbered. They are probably short. Behave as you would want to see yourself when you look back. Because you're going to be looking back at these decisions for an awfully long time.