Sovereignty disputes in the East Sea are not a new problem as it was started since World War II and escalated during the Cold War. But why the territorial disputes is higher now than ever before?
There are many reasons why the tension is high now. The importance of rocks and islands has increased since 1982 because of the impact of the UN Convention on the Law of the Sea. UNCLOS gives rights to resources within certain distances of land features. If a country controls a rock or island it also has the right to the resources for as much as 200 nautical miles around it. That has made it important for claimants to occupy the features.

Tension rose again after May 2009, which was the date that the PRC submitted a copy of its U-shaped line map to its claim with the UN Commission on the Limits of the Continental Shelf. This was the first time that China had officially used the map in an international setting. It alarmed the rest of Southeast Asia.
China has been asserting its claim in the South China Sea in more and more assertive ways – cutting the cables of oil survey vessels in 2011, deploying an oilrig off the Paracels in mid-2014 and developing reefs in the Spratly islands into naval bases at the moment.
In previous decades this might have been considered only a regional problem but it is taking place at a time of growing competition between the US and China for leadership in Asia. The US has major strategic interests in Southeast Asia: trade, freedom of navigation, the rule of international law and the security of its treaty allies. Therefore anything that is perceived to increase China’s position in the South China Sea is seen to damage the United States’ interests and so the disputes take on a global dimension. Some of this is correct, some is exaggerated.

What policies or preventative measures might reduce the possibility of military conflict in the region? What are the major obstacles that are currently preventing this reduction?
The states around the South China Sea need to base their claims to the features in proper, verifiable evidence rather than in a sense of entitlement. For example, China’s claim to the James Shoal is clearly nonsense since it is based upon a translation error in 1935. China is actually claiming territory that does not exist – the James Shoal is underwater!
All the claimants should stop asserting their claims to wide areas of ocean or entire archipelagos and, instead, frame their claims to specific features in the sea. If this were the case then the ROC would probably have the strongest claim to Itu Aba (Taiping Dao), Vietnam to Spratly Island (Truong Sa Lon), the Philippines to Thitu (Pagasa), Malaysia to Swallow Reef (Layang-Layang) and then they can argue about the others.

The US Pacific Fleet regularly sails through the South China Sea as a reminder of its power to assert free trade in the waters

China will continue to "ignore" the concerns of the neighbors in the South China Sea and the use of economic power as leverage in relations with these countries, so do you think have a way forward to resolve the disputes and preserve the peace of Southeast Asia? And  what ASEAN should be do to develop a Code of Conduct, including strict and concrete legal stipulations?
I doubt if there will ever be a meaningful Code of Conduct between ASEAN and China for the South China Sea. Why would China accept limits on its behaviour? Who would enforce the CoC in case of violations? ASEAN has three options - to keep talking in the hope that something might change - to abandon the talks - or to sign up to yet another positive-sounding document that has no teeth. I don’t think that ASEAN will want to formally abandon hope of progress with China, so it’s up to Vietnam and the Philippines and perhaps the other ASEAN claimants to decide whether they want to keep on talking or agree something called a Code of Conduct that doesn’t actually change anything. I think that they’ll keep on talking for now.
Economic leverage can work two ways. Yes China is offering plenty of incentives for countries to work with it. But ASEAN is not being very receptive to those advances. Last year China announced a ‘Maritime Cooperation Fund’ – but it doesn’t seem to have actually done anything. The much-hyped ‘Maritime Silk Road’ doesn’t appear to amount to anything beyond some subsidies for Chinese businesses. I think ASEAN countries are wary of China’s moves and aren’t cooperating.