This paper is written to analyze the impact of controversies over the deployment of THAAD (Terminal High Altitude Area Defense) on South Korea-U.S. alliance. For this purpose, it uses "Autonomy-Security Trade-off Model" and determines whether the controversies stemmed from the truth or rumors. As a result, this paper found that the controversies over the THAAD deployment could have weaken the South Korean alliance with the U.S., because South Korea was not active enough in giving up autonomy regarding the issue. South Korea considered the oppositions of China on the deployment too much and could have raised the suspicion by the U.S. regarding the strength of the alliance. To make matters worse, the controversies turned out to be stemmed from rumors instead of truths. The THAAD cannot intercept Chinese ICBMs, cannot detect the Chinese military activities and need not to be payed by South Korea, contrary to the rumors. The electro-magnetic waves from the THAAD radar is not as harmful as described in South Korean media. The deployment of THAAD itself is important to the South Korea-U.S. alliance. However, the process of the decision of South Korea for the deployment could be more important to the alliance.