National police spokesman Chhay Kimkhoeun immediately replied that Sam Rainsy would be arrested on arrival to face criminal charges.
Sam Rainsy’s Cambodian National Rescue Party (CNRP) previously sought to have international observers accompany him on his return, but they were denied visas.
Cambodia’s judicial system is entirely controlled by the strongman government of Prime Minister Hun Sen.
November 9 marks the 66th anniversary of Cambodian independence from France.
Eng Chhai Eang, a CNRP spokesman, said the date “represents the freedom for Cambodians to decide our future fate for peace and prosperity”.
“It’s just about the timing, as Cambodians everywhere are ready to step up their efforts in our fight for a positive change that benefits the whole population,” he said.
“We also believe that our return to Cambodia during this time will help mend the political crisis, which may impact the EU’s decision on whether to withdraw trade preferences for Cambodia.”
Sam Rainsy, 70, left Cambodia in late 2015 in the face of what he condemned as politically motivated convictions for defamation and other charges.
In June, the former opposition chief said he would return by September, but he appears to have pushed that date back.
Although Sam Rainsy is still probably Cambodia’s most popular political figure, his years in exile have undermined his credibility after he has vowed several times to return and then failed to appear in Phnom Penh.
In 1997 Sam Rainsy was the apparent target of an assassination attempt when grenades were thrown at a small protest in Phnom Penh. At least 16 people were killed and many others wounded.
The CNRP, which had been growing in popularity in a series of elections, was dissolved by a court order in November 2017, ahead of the general election in July 2018. The ruling Cambodian People’s Party went on to “win” all 125 seats in the National Assembly.
The CNRP’s co-founder, Kem Sokha, was charged with treason, based on his links to a US-based, pro-democracy NGO, and is currently under house arrest.
In response to Cambodia’s crackdown on democracy, the US imposed visa sanctions on senior government figures. The European Union launched a six-month study that ended this week to decide whether Cambodia should continue to qualify for tax-free access under the Everything But Arms trade agreement.
Brussels is due to publish its report in November and make a final decision in February 2020 on Cambodia’s trading status. Any tariff suspension would come into effect by August next year.
The Garment Manufacturers Association of Cambodia said the loss of EU access would hit nearly 4 million Cambodian clothing makers and their families.
Picture credit: Wikimedia