Donald Trump doesn't know much about foreign policy, nuclear policy or the world generally: Barack Obama

File image of Barack Obama. PTI[/caption]Obama described the US nuclear umbrella for Japan and South Korea, in place of their own arsenals, as "one of the cornerstones of our presence in the Asia Pacific," which has provided the US peace, prosperity and flowing commerce, CNN reported."It has prevented the possibilities of a nuclear escalation and conflict," he said, adding "You don't mess with that. It's an investment that rests on the sacrifices that our men and women made" in the Second World War."We don't want someone in the Oval Office who doesn't recognise how important that is," Obama added.The summit came as the Republican front-runner to replace Obama in the White House made several controversial nuclear proposals this week.Trump said that nuclear proliferation is the world's biggest challenge, but also suggested at a CNN town hall on Tuesday that it may be time for Japan and South Korea to develop their own nuclear arsenals so the US can pull back from Asia.Trump has also suggested re-drawing US security relationships in other regions, arguing that Germany and Saudi Arabia need to do more in their own defence or pay the US more for the protection it offers.The summit, the final of four Obama has held during his presidency since 2010, drew over 50 leaders from around the world to discuss ways to prevent the use of nuclear weapons and better secure nuclear materials, especially from the threat of nuclear terrorism.Obama said global efforts to improve nuclear security have removed from circulation material that is equivalent to 150 nuclear weapons, safeguarding it from extremists.