By deploying its Terminal High-Altitude Area Defense system in the Republic of Korea, the United States will harm regional peace and stability, and put Seoul in a precarious position. The crux of the Barack Obama administration's "pivot to Asia" strategy is to consolidate its military presence in the region and THAAD is an important part of that move.

Apart from the ROK, the US also hopes to deploy the missile system in Guam, Japan and the Philippines. But since deployment of THAAD in the ROK was considered the most difficult to achieve, the Obama administration sees Seoul's nod as a key step forward in its "pivot to Asia" strategy and wants to complete the process before ROK President Park Geun-hye's term in office is over.

Donald Trump has said the ROK should be responsible for its own security, and this attitude of the Republican presidential nominee has been worrying Seoul. When the US withdrew parts of its forces from the ROK between 1950s and 1970s, Seoul felt "unsafe". So now, given Trump's stance, the Park administration seems eager to secure US protection by opening the door to the missile defence system. The deal could even include the introduction of US tactical nuclear weapons in the ROK arsenal in the future.