Coup leader first Muslim army chief
Despite surge of anti-government protests in recent months, Sonthi had given indications that a government seizure wasn't in the cards.
"Military coups are a thing of the past," he said recently, echoing comments from other top military officers. "Political troubles should be resolved by politicians."
Sonthi, the first Muslim army commander in Buddhist-dominated Thailand, was appointed to the army's top post last year with a mission to deal with an Islamic insurgency in the country's south.
He was seen as having unique qualifications for the job. His religion gave him extra credibility among Muslims, and his service as a former head of the special warfare command made him well suited to the task. In addition, his service in the Vietnam War won him the respect of veterans.
"I will make the Royal Thai Army into the army of the people, and will make soldiers the beloved soldiers of the people," Sonthi said as he assume the top post last year.
Coups are nothing new to Thailand, but many hoped that after 14 years of uninterrupted civilian rule, the days of revolving door military regimes might finally be over.
In recent months, however, mass protests and an impasse over flawed elections have thrown the country into its worst crisis since the last army takeover in 1991.
Even as recently as last week, amid growing tensions in the wake of an alleged bomb plot against now-ousted Prime Minister Thaksin Shinawatra, Sonthi was quashing coup rumors.
"Has the situation gone to that point? No. There is still a way to go by democratic means," the 59-year-old officer said then. "We should stop talking about it. It is impossible."
Sonthi is thought to be close to Thailand's revered monarch, King Bhumibol Adulyadej, who has expressed unhappiness with Thaksin's administration.
When the move came Tuesday against the prime minister, he was in the forefront.
Sonthi took power without a shot being fired and coup officials said he will serve as acting prime minister.