Former prime minister Thaksin Shinawatra has come back to Thailand after 15 years in exile, just hours before a referendum that will determine the next leader of the country.

Conservative royalists have long feared Thailand's most successful elected leader, supporting military coups and contentious court cases to undermine him.

Now, however, the politically ambitious telecoms magnate is back, presumably after striking an agreement with the men who overthrew his party in 2014 to keep him out of jail.

He is serving sentences of up to ten years for crimes he claims were politically motivated.

He arrived in Bangkok's capital at 9:00 local time (01:00 GMT) via Dubai and Singapore on a private aircraft. Hundreds of his ecstatic supporters cheered, made speeches, and sang as Mr. Thaksin eventually fulfilled the numerous promises he made to return. 

He was transported to the Supreme Court, where he was sentenced to ten years in prison, and then to Bangkok Remand Prison. 

Mr. Thaksin's Pheu Thai party is anticipated to join a coalition government later today - a convoluted process that has brought Thailand full circle in three months.

It began with the radical, youthful Move Forward party, which won the most seats in the May election, leading the charge for a new dawn.

Move Forward initially partnered with Pheu Thai, but it is now certain that the coalition will include almost everyone but the reformers, including two parties led by former coup-makers - a deal with its sworn enemies that Pheu Thai pledged not to make.

Pheu Thai asserts that the two events are unrelated. Few individuals hold this view.

It is true that Pheu Thai's hands have been constrained by the unelected senate, a 250-seat constitutional landmine installed by the military junta that ruled Thailand for five years following the 2014 coup.

The junta-appointed senators are permitted to vote alongside the 500 elected MPs for the new prime minister.

Their mission is to obstruct any party that might threaten the status quo - the monarchy, military, and large business alliance that has dominated Thai politics for decades.

Inexplicably, they refused to support the Move Forward-led coalition with Pheu Thai, despite the coalition's overwhelming majority in the lower chamber.

When it was Pheu Thai's turn to reach an agreement on a new coalition, its need for senate support necessitated the incorporation of a number of its erstwhile adversaries.

Any minority government formed without Pheu Thai and Move Forward would soon fail, as senators cannot participate in normal parliamentary votes on issues such as the budget.

The Pheu Thai leadership was impatient and invited the ultra-royalist United Thai Nation to join the coalition, whose leaders have been harshly critical of the Shinawatra family and their supporters and helped overthrow Thaksin's sister Yingluck's government. 

In the end, the ultra-royalists' long-running feud with the Shinawatra family was overshadowed by the perceived threat posed by Move Forward and a younger generation of Thais who demanded a discussion about the monarchy's power and fortune.

However, some members of Pheu Thai are appalled by the cynical pragmatism of this agreement.

They warn that the party will lose even more of its formerly fervent grassroots adherents and, possibly for good, its two-decade-long hold on Thailand's electoral politics.