If ever there was a definition of a 'known unknown' ? la Donald Rumsfeld, it is modern Myanmar. Even hardened sceptics across the region acknowledge surprise at the pace of changes now unfolding, from the release of political prisoners to open campaigning by opposition leader Aung San Suu Kyi. Of course, no one knows precisely how the reform drive is going to play out, and just how far it will be allowed to go under its military-backed civilian government. Underpinning such uncertainty are other nagging questions - why now? What is the catalyst that pushed Myanmar's military to accept change after so many years of darkness? As veteran US senator John McCain said recently after his visit to Myanmar: 'I've puzzled over that.' He is not alone. It is a question that echoes across the region's staterooms. Could it be that the leadership craves broader international relations after years of relative isolation? Or have Western sanctions finally taken a toll? Or perhaps it was the fear of an Arab-spring-style uprising. The junta's grip on power, after all, has long been tenuous and maintained through crude use of the jackboot, as it showed when it shot unarmed monks in 2007 to put down the last spell of significant unrest. It has always lacked the systemic control, and repression, of a communist state. Then there has been the largely unheralded role of Asean. The Association of Southeast Asian Nations courted international controversy by allowing Myanmar to join in 1997, yet over recent years it has been crucial in dragging the regime towards the light, quietly stressing the need to change and giving its leaders the confidence to take the initial steps. What is driving change is an important question. The answers will provide clues to how deep reforms may run. It also reminds us that, despite those good vibes, there is still so much the outside world does not yet know about the way Myanmar works. The military backs the nominally civilian government led by Thein Sein, who is a former general. Significantly, it still holds constitutional veto power through its guaranteed block of seats in parliament. Unusually, Thein Sein's position as president does not make him commander-in-chief of the armed forces. 'For now the military is being quietly supportive by being quiet,' said one veteran Asean envoy in Yangon. 'They are showing no appetite for involvement in parliament, despite their seats ... for now, that is allowing reforms to take root and evolve. We have to hope that situation remains ... But it can only be a hope, and that is the trouble.' Thein Sein himself has spoken of the need to forge genuine peace and stability through reform and economic development, but has said little about the precise turning point for the old regime. He has, however, insisted there will no turning back. Only the pace has to be determined. Suu Kyi recently stated that significant government elements 'genuinely desire reform'. 'If we wait for solid guarantees, we can never proceed,' she warned, stressing that risks had to be taken. They are risks the whole region has a stake in. Greg Torode is the Post's chief Asia correspondent.