By Andrew Walker
US President George W Bush's trip to South-East Asia comes exactly 100 years after the first presidential overseas visit.
The intervening time has seen the initial small-scale journeys grow into a fully-blown royal progress.
Early images of presidential visits have become iconic
An extraordinary man, the 26th president.
A sickly child, Theodore Roosevelt developed into the living embodiment of swaggering machismo.
He led his Rough Riders in a near-suicidal charge up San Juan Hill during America's 1898 war with Spain, an action for which, in 2001, he posthumously received the nation's highest award for military valour, the Medal of Honor.
But his abilities stretched to far more than fighting.
The man who once said "speak softly and carry a big stick" was awarded the Nobel Peace Prize for helping end the 1904-5 Russo-Japanese War.
And, 130 years after the signing of the Declaration of Independence, Teddy Roosevelt became the first president to travel outside the country while in office, visiting Panama to inspect the progress of building the strategically-crucial canal.
It seems extraordinary today that the "leader of the free world" should not venture abroad but, in November 1906, both the presidency and technology were hugely different from what they are today.
In 1906, just three years after the Wright brothers took to the sky at Kittyhawk, air travel was still in its infancy.
Steamships - stately, fabulously well-equipped but slow - were the preferred mode of travel for international statesmen, who communicated over the telegraph.
Lyndon B Johnson boosts troop morale in Vietnam
By World War II, airliners ferried Franklin D Roosevelt around the world, to meetings with the major powers at Casablanca, Cairo, Tehran and Yalta.
But these were secret occasions, designed to drive the war effort, not to engage with the public.
Roosevelt's successor, Harry S Truman, travelled rarely.
Indeed, in his eight years at the White House, he ventured abroad on only six occasions.
Modern presidential tours can really be said to have begun with Dwight D Eisenhower.
In nearly 40 visits, he travelled everywhere from Afghanistan and Taiwan to Chile and the Vatican.
John and Jackie Kennedy - stylish, understated, intellectual - provided a counterpoint to antediluvian Soviet leaders like Nikita Khrushchev.
JFK's "Ich bin ein berliner" speech from the Berlin Wall provided both the Cold War's most famous catchphrase and crucial reassurance to a Europe fearful of US disengagement in the face of the Red Army.
And Kennedy's visit to his ancestors' village in Ireland, just months before his death, paved the way for later pilgrimages by his successors.
Images of Richard Nixon transformed him into a political visionary
But if Kennedy brought glamour to the whole business of presidential travel it was Richard Nixon - dour, driven, some would say paranoid - who pulled off perhaps the greatest of all US diplomatic coups, visiting both the Soviet Union and China.
One of America's most noted presidential scholars, Professor Michael Genovese of Loyola Marymount University, is clear what the biggest difference is between those earlier presidential visits and today's: television.
"Nixon was the first president to truly exploit the presidential trip in an age of television, as the president became a celebrity as well as a politician," he says.
"Nixon's trips to China and the Soviet Union as well as to Europe and the Middle East were the first truly staged events designed for maximum television effect."
Major events were scheduled so they could be broadcast on primetime television back home in the States, cameras captured the chief executive in stunning locations like the Great Wall of China, the whole effect being to portray Nixon as a man of action, a charismatic diplomat and, above all, a political visionary.
Presidential visits have provided many other memorable moments, like Bill Clinton being mobbed by crowds in Belfast in December 1995, and his emotional words to the people of Northern Ireland: "You must say to those who still would use violence for political objectives: 'You are the past; your day is over.'"
Thousands marched in protest against George Bush's visit in 2003
There was also Ronald Reagan on the Normandy beaches for the 40th anniversary of the D-Day landings, Lyndon B Johnson with US troops in Vietnam, and Jimmy Carter's controversial visit to Tehran on New Years' Eve 1977, where he praised the Shah and the country he called "an island of stability".
Professor Genovese has seen a radical shift in the nature of presidential trips.
"The media covers presidential trips as imperial visits of a kingly variety, and this in turn creates the impression that the president is the government," he says.
"As nothing is left to chance, these trips are carefully orchestrated for maximum effect."
President Bush's visit to London in November 2003 was a case in point.
Travelling with his own bullet- and bomb-proof limousine, a mobile operating theatre and armoured helicopter, the president was cocooned in a vast exclusion zone, designed to foil both terror attacks and mute anti-war protestors.
The White House had demanded the banning of all protest marches and the virtual closure of the centre of the capital.
In the event, 250 armed US Secret Service officers patrolled the UK capital: all were authorised to use their weapons.
Such high security, and the president's lack of contact with people on the street, left Mr Bush's many critics unconvinced by his keynote speech, in which he talked of seeking "the advance of freedom and the peace that freedom brings".