In a comment to my post of July 16 about the THAAD deployment in Guam being made permanent, a question was raised about why THAAD was proposed for South Korea and Aegis Ashore for Romania and Poland (and why not vice versa).
There are two main technical issues that almost certainly drove the decision of which system went where:
(1) Europe can be almost completely covered by two Aegis Ashore sites but achieving similar coverage with THAAD would require a prohibitive number of THAAD batteries. On the other hand, S. Korea is small enough to be covered by one or two THAAD batteries.
A single Aegis Ashore site (with the Block IIA interceptor) can cover a much larger geographical area than a single THAAD deployment. The Block IIA interceptor is scheduled to begin deployment in 2018. This larger coverage area occurs because the Aegis Block II interceptor has a much higher burnout speed (likely about 4.5 km/s) than a THAAD interceptor (likely about 2.6-2.8 km/s) and thus can reach out to make intercepts at much greater ranges.
This is illustrated in two 2007 Missile Defense Agency Briefing slides. The yellow “footprints” in Figure 1 below shows the area that could be covered by three THAAD batteries in eastern Turkey against Iranian ballistic missiles. For THAAD, this situation — in which the attacking missiles are launched from a country bordering the country targeted – is closely analogous to the North Korea-South Korea situation. However, the three THAAD batteries together cover only a small fraction of Turkey.
Figure 1. Coverage of Europe against Iranian ballistic missile by THAAD, Aegis (Block IB), and two-stage GBI interceptors. Slides from MDA Executive Director Patricia Sanders, “Missile Defense Program Overview For The 4th International Conference On Missile Defense,” June 26, 2007. Available at: https://mostlymissiledefense.files.wordpress.com/2013/06/bmd-overview-sanders-june2007.pdf