In the name of tackling missile threats from North Korea, the South Korean government finally decided to deploy the THAAD system in Seongju County, North Gyeongsang Province over 250 kilometers southeast of Seoul. Since the missile range of the THAAD system is about 200 kilometers, which leaves out the Seoul metropolitan area, the South Korean military will deploy PAC-3 defense system, probably in 2017, to defend Seoul. Interestingly, both THAAD and PAC-3 are manufactured by Lockheed Martin, the largest single contractor for the US government.

Some of the Korean public however have different thoughts over the deployment. Although national polls show that over 70 percent of South Koreans support THAAD, this varies dramatically by province. "We oppose THAAD with our lives," is how local residents from Seongju expressed their anger in a rally after the announcement of the location. The locals seem to have plenty of reasons to say "No" to Seoul's overall decision regarding the THAAD system.

Meanwhile, the deployment of the THAAD system may bring some impacts to US-South Korea alliance and even to South Korea's diplomacy in the long term.

South Korea has long been tiptoeing between the US and China. This is an unstable triangle framed by two solid lines (China-South Korea economic ties and the US-South Korea military alliance), and one "dotted" line (China-US strategic competition).

Framed in such a difficult regional puzzle, South Korea definitely needs more strategic pivots to balance itself against the "tidal effects" brought by a rising China and the US "return" to the region, and this can be one of the main rationales for South Korea's adoption of its well-known "middle power" diplomacy.

However, the deployment of THAAD may bring certain revisions to the US-South Korea alliance, and it may even cause some uncertainties to South Korea's "middle power" dream in one way or another.

As the deployment not only symbolically but also substantially further re-consolidated the US-South Korea alliance, which has been undergoing transitions in the past decades, it may  limit South Korea's diplomatic space in East Asia, particularly with China. There may be a strengthened alliance between Seoul and Washington after the deployment of THAAD and PAC-3 afterward, but the situation may be more complicated and delicate.

For some time, the US-South Korea alliance has been pictured as a relationship built on "shared values, new frontiers" with cross-regional and global significance. But at this particular moment, the US once again successfully pressurized South Korea with the alleged missile threats from the North, and this may remind us of South Korea's still delicate role in its alliance with the US. The deployment of THAAD definitely concerns China's security, and the consequences may be rather negative.

In view of the previous brief honeymoon with China, the deployment of THAAD can also be understood as South Korea taking two steps backward after one step forward. Previously, China may have practically acquiesced in a US-South Korea alliance near its doorstep for some strategic and political considerations. But after the deployment of THAAD, China naturally needs to reconsider its policy of putting up with a crouching tiger nearby.

However, its reactions do not necessarily include strong economic retaliatory measures against South Korea, and Seoul may have already been on track to further corner and marginalize itself in the unstable triangle.

Needless to say, in a game among two great powers and one small power, it can be very unwise for the small to be targeted by either of the other two.

Overall, the current situation over THAAD suggests that a seemingly strengthened alliance may actually put Seoul in a corner, and even play down South Korea's long-term "middle power" diplomacy.

The author is a research fellow at the Yonsei Institute for Sinology and a lecturer at the Graduate School of International Studies, Yonsei University in South Korea. opinion@globaltimes.com.cn