By Helen Murphy and Luis Jaime Acosta
BOGOTA (Reuters) - Colombian President Juan Manuel Santos, who staked his legacy on ending a five-decade war with Marxist rebels, leaves office next week satisfied he oversaw a historic peace deal but frustrated he could not do more to unite the nation and reduce inequality.
Internationally lauded for negotiating peace with the Revolutionary Armed Forces of Colombia (FARC), Santos has been criticized by many Colombians who say he sold out to the rebels in return for a Nobel Peace Prize and made the country more dangerous.
But Santos, 66, says he sleeps soundly at night, proud of silencing FARC weapons and beginning negotiations with the National Liberation Army (ELN), Colombia's remaining rebel group.
"I've got more grey hair now and I'm a grandfather, but I'm calm and very satisfied with the results," the two-term Santos told Reuters before his handover to right-wing President-elect Ivan Duque on Aug. 7.
"Politically I feel a little frustrated, I would have liked to leave the country more united. Believe me, I tried, but it wasn't possible."
The scion of one of Colombia's most prosperous families, Santos had not been expected to spearhead a peace process with the FARC, which battled a dozen governments during a conflict which killed over 220,000 people and displaced millions.
He had been one of the nation's toughest defence ministers under hard-line former President Alvaro Uribe, meting out some of the harshest blows against the group and pushing them deeper into the wilderness before bringing them to the negotiating table.
His decision took Uribe by surprise and turned him into a bitter enemy who accused his former protégé of betraying FARC victims and failing to jail war criminals. Uribe charges the FARC deal opened the way for new crime gangs to set up.
The onetime allies spent years in public spats that set the powerful Uribe and his party against the peace process, almost hobbling it completely when Santos called a referendum on the final deal.
The vote's failure became Santos' biggest political crisis and shock.
"That was a bucket of cold water that affected us all," said the former journalist, who pushed a renegotiated deal through congress in 2016.
Duque, also mentored by Uribe, has promised to make changes to the peace accords to jail former FARC commanders who committed crimes.
But Santos said it will be almost impossible to change the accords, which have been endorsed by the international community and broadly supported by the constitutional court.
"Peace can't be changed for many reasons and for ethical and moral reasons - nobody wants to go back," said Santos, who beat prostate cancer while in office.
Though peace is the most enduring part of his presidential legacy, Santos also oversaw a fall in unemployment and a decline in poverty.
"We were the Latin American country that most cut inequality, but, still, inequality here is shameful," he said.
Santos also changed the distribution of oil and mining royalties and made it possible for land seized by right-wing paramilitaries and rebels to be returned.
But his time in office has also been marked by corruption allegations, and he was forced to apologise after revelations his 2010 and 2014 campaigns took funds from disgraced Brazilian construction company Odebrecht.
Santos plans to retire completely from politics, write a book and use his Nobel Prize to travel the world on lecture tours extolling peace.
(Reporting by Helen Murphy and Luis Jaime Acosts; editing by Jonathan Oatis)