The US president and first lady, at the king’s invitation, are on a state visit to the UK and will stay at Windsor castle. The event is laden with ceremony and glitz, but it also carries great political potential – for the host nation in particular.Formal visits by foreign heads of state are generally aimed at strengthening international relationships. The invitation for this visit was handed to Donald Trump in the Oval Office, on camera, by Keir Starmer at a time when the British prime minister was seeking to act as the bridge between the US and Europe over the war in Ukraine.While many countries have state visits, their ceremonial style varies. In the US, ceremonial honours usually involve arrival on the White House south lawn, accompanied by a military band, 21-gun salute and then a state dinner. But it is the scale and grandeur of the UK’s carriage processions, state banquets, speeches in parliament and military pageantry, tied to the monarchy’s long history, which make them a powerful diplomatic tool for the UK. Trump has made no secret of his delight at being invited for an unprecedented second state visit. Trump’s praise of the royal family is testament to the soft power at work here. An offer of an audience with the king can deepen diplomatic ties. In this instance, it’s a powerful tool for enhancing the UK-US relationship at a time when this is a priority for Britain. How state visits workIn the UK, the procedures surrounding state visits are guided by conventions (traditions) and protocols. The prerogative – the government’s residue discretionary power – also comes into play.The formal invitation for a UK state visit is issued in the name of the king, as head of state. However, in practice, the decision as to who receives them is usually based on another country firstly expressing an interest, and then the UK government offering ministerial advice to the king to extend an invitation. This advice usually comes from the foreign secretary in conjunction with the prime minister, as part of their prerogative powers surrounding foreign relations. Most of Trump’s visit will take place at Windsor castle. Shutterstock/Tomas Marek Once an invitation is issued, the Foreign, Commonwealth and Development Office, together with the royal household, will set the agenda for ceremonies, banquets, guards of honour and meetings with the king and prime minister.Typically, the king will receive two foreign heads of state per year. In 2024, he hosted the emperor and empress of Japan, and the amir sheikh and sheikha of Qatar. These visits are usually only a couple of days and start with a ceremonial welcome attended by the king and other senior royal family members.Having arrived in London on Tuesday, September 16, Trump will spend Wednesday at Windsor castle with the royal family and attend a state banquet in the evening. He will leave on Thursday for Chequers, the prime minister’s country residence, where the two will hold a bilateral meeting. Diplomatic immunity, costs and threat levelsTo facilitate secure visits, the State Immunity Act 1978 affords heads of state the same privileges as are applied to the heads of diplomatic missions. The visits are further underpinned by customary international sources such as the Vienna Convention on Diplomatic Relations of 1961. While these do not make explicit reference to state visits, they establish practices surrounding immunities and the treatment of heads of state in the discharge of their public duties.In reality, hosting the US president means the UK has to manage a high threat level – which also means paying a lot, mostly for security and policing. Trump’s last state visit in 2019 cost £3.9 million.But state visits bring reward as well as expenditure. These are not just opportunities for ceremony: meetings take place around the pomp, and there are always vigorous diplomatic efforts to reach agreements that can be announced while leaders are delivering speeches during the visit.Even before Trump had touched down, it had been announced that Google would invest £5 billion in artificial intelligence in the UK over the next two years. More announcements of this kind can be expected to follow. This will go some way to meeting a call from the UK parliament’s Business and Trade Committee for Starmer to apply “maximum pressure” on Trump to secure a trade and technology alliance to rival China. Starmer will also be hoping to negotiate on tariffs during the Chequers meeting on Thursday. What the king doesThe UK is a constitutional monarchy, meaning the king’s powers are limited by law, and are largely used by the UK government rather than the royal family. While the king is able to “advise and warn”, decisions are ultimately taken by the government. In that sense, the king reigns, but does not rule.Nonetheless, the monarchy is very useful in delicate diplomatic situations. Wielding significant soft power in international diplomacy and serving as a visible symbol of stability and continuity on the global stage as head of the Commonwealth, the king can promote shared values and cooperation across borders.His role in awarding honours, celebrating the voluntary and charitable sector, and delivering speeches often makes him synonymous with officially recognising success and excellence. The demands made of the monarchy to remain politically neutral on all matters can lead to them being seen as a unifying force.It is noteworthy that even during moments of intense geopolitical tension, such as after the Russian invasion of Ukraine in 2022 when significant economic sanctions were being placed on Russia by the UK, the Russian president, Vladimir Putin, still offered condolences to the new king upon the death of Queen Elizabeth II – describing it as a “heavy, irreparable loss” of an “authority on the world stage”.In this context, the king’s capacity to act as a bridge in diplomacy is an asset. Starmer will be hoping that the cost of hosting this second state visit will translate into tangible outcomes for the UK.Stephen Clear does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.