A big election victory never looks more impregnable than on the morning after. The Conservative triumph on 12 December was no exception: the verdict seemed absolute. Boris Johnson hymned the dawn of a glorious new era. Labour took refuge in the infantile disorder of imagining itself the heroic “resistance” to an all-powerful ruler. The headlines promised that a Commons majority of 80 would mean “Johnson unleashed” with a mandate “to govern as he likes”.

Two months on these can be seen as silly overstatements. The whirligig of time is already threatening revenges on the Johnson government. The most important example of this is its confusion over the role of Huawei in building the UK’s 5G phone networks. But Huawei is one dilemma among many. It is also an example of a larger truth. Just as all political careers are said to end in failure, so do all governments.

For governments, the morning after the election is as good as it gets. From that day on, things get tougher. The arrow of time tends to fly from order towards disorder. A form of political entropy sets in, almost as remorselessly in government as in the laws of physics. To govern on day 100 or day 1,000 as you govern on day one is folly. Goethe knew what he was doing when he made Mephistopheles tell Faust that he would lose his freedom as soon as he asked for the perfect moment to last for ever. It seemed like a great deal, but it was the opposite.

This worm in the bud exists for Johnson too, even this week. Britain’s exit from the European Union is unquestionably Johnson’s victory. It will be his principal claim on history. But then what? Once the departure is done, it is done. Johnson will try to trade on it for years to come, and he may do so quite successfully. But he now owns Brexit outright: he will own its failures, delays and compromises as well as any successes. Whatever happens in the coming stages of the Brexit process, things will get more, not less, tricky.

Johnson still possesses enviable political capital. But he has had to spend a fair amount of it over Huawei. Many of his MPs, and some of his cabinet, think the decision to let Huawei build parts of the 5G network is seriously wrong. Some of them will go on saying so, mainly in private, but some – especially some who have given up hope of ministerial office – will so do in public. An impressive list of big Tory names attacked the Huawei decision in the Commons on Tuesday: Liam Fox, Julian Lewis, David Davis, Iain Duncan Smith, Penny Mordaunt, Bernard Jenkin, Damian Collins.

A majority of 80 gives Johnson a big cushion, but he should not be complacent. National security is an issue on which many Tories, especially those with an Atlanticist or military background, have gut instincts that will not change. Some, even now, are already on manoeuvres for the future. Others have been backbench rebels for so long that they can’t kick the habit. But it will only take a monstrous cyber-hack or a piece of heavy-handed action by the Beijing government against Hong Kong, Taiwan or the Uighur Muslims for the whole Huawei issue to flare up again.

Even at this early stage in the life of the new government, it’s not just Huawei that presents dilemmas. At prime minister’s questions on Wednesday, Johnson had to tiptoe around the issue of Donald Trump’s Middle East plan. As with the issue of Iran in the wake of the Qassem Suleimani assassination, Johnson has a difficult balancing act to perform. He has to juggle the Trump connection with longer term issues about trade, security and alliances. He has to weigh the response in the Tory party, the country and the EU. And he has to make his response to each of these dilemmas fit somehow within the government’s fatal conceit of post-Brexit Britain as a freebooting global power.

Difficult though they are, not even Johnson thinks these international jugglings are as important as the ones he faces at home. Here the list is also very long and very live. The most immediate is HS2, on which a decision that will again divide the Tory party may come on Thursday. Rail privatisation, in the wake of Wednesday’s renationalisation of Northern Rail, is not far behind as a difficult issue. Heathrow expansion looms. Meanwhile issues such as Northern Ireland, Scotland, immigration, fishing, the budget and the cabinet reshuffle will all have the power to knock the government off course too.

This is not to pretend that Johnson remains anything except dominant in current politics. His authority remains largely untapped. Nor is he powerless to shape his own political destiny if things go awry. But it is to say that politics is quickly reconfiguring itself in a familiar way that is at odds with the simplistic assumptions of December.

Modern British history contains many examples of governments that were brought low in spite of winning clear majorities in elections. Harold Wilson’s 1966 government did not survive devaluation and the failure to reform the trade unions. Margaret Thatcher’s victories in 1983 and 1987 were torpedoed by Westland (the Huawei of its day) and the poll tax respectively. John Major’s win in 1992 was kneecapped by a run on the pound and by Europe. Tony Blair’s 2001 landslide was wrecked by Iraq. David Cameron destroyed himself over Brexit.

Some of these governments were strong enough to weather those challenges. Others were not. Johnson is lucky that so much depends on the credibility of the opposition. His government may prove to be one of the more resilient ones. But no perfect moment endures. Political danger never goes away entirely. Even in this week of his greatest triumph over Brexit, it is not true to say Johnson is unleashed. In fact the leash that restrains him is becoming increasingly obvious.

• Martin Kettle is a Guardian columnist