Assad now stands accused of killing civilians with chemical weapons and barrel bombs during Syria's civil war.
President Xi Jinping of China:
Protesters turned out in force for his visit in October 2015 – and there were even rumours of royal unrest.
Amnesty International, pro-Tibet campaigners and followers of the Falun Gong spiritual practice, banned in China, were present on The Mall in central London.
President of China Xi Jinping and his wife Peng Liyuan meet the Queen and Prince Philip, as they arrive for a state banquet at Buckingham Palace in October 2015. Photo / Getty Images
Vladimir Putin:
The Russian president's only state visit in June 2003 was marked by protests against his repression of separatists in Chechnya.
The visit, organised by Tony Blair, who admired Putin as a "moderniser", took place three years before the murder of Russian dissident Alexander Litvinenko in London, allegedly by Russian agents, which hardened relations with Moscow.
Russia's President Vladimir Putin and the Queen as they arrive for a banquet at Spencer House in London, in June 2003. Photo / Getty Images
Robert Mugabe of Zimbabwe:
The then-president was blamed for the massacre of 10,000 people in the 1980s, long before his visit to Britain in May 1994.
But, with the full horror yet to emerge, he was welcomed as a trading partner and for his "influence on peace" in Africa.
He was greeted by Princess Margaret at Heathrow and taken by Rolls Royce to meet the Queen and then-PM John Major.
He accompanied the monarch for a five-minute procession by carriage to Buckingham Palace.
Mugabe was made an honorary KCB during the visit. He was later stripped of the honour.
Robert Mugabe, then President of Zimbabwe, is greeted by the Queen and Prince Phillip at Buckingham Palace during his state visit in May 1994. Photo / Getty Images
President Suharto of Indonesia:
His regime was blamed for up to a million deaths after the invasion of East Timor, four years before his UK visit in November 1979.
But the despot's arrival was hailed by some as an "excellent opportunity" for the UK defence industry. He was confronted in Downing Street by protesters.
President Nicolae Ceausescu of Romania:
Dissidents from Romania who had fled the Communist regime were outraged at his visit in June 1978.
Jim Callaghan's Labour government wanted to broker a deal with Romania to co-produce aeroplanes.
Shortly before their arrival, French president Valery Giscard d'Estaing telephoned the Queen to warn that Ceausescu and his wife Elena had walked off with valuables from their state rooms on a visit to Paris.
President Mobutu Sese Seko of the congo:
The dictator, who visited in December 1973, presided over the collapse of his country's economy and locked up dissidents.
He was later found to have embezzled £12 billion ($17 billion NZD), and was blamed for the deaths of 230,000 people.