A U.S. military video shows the arrival of Terminal High Altitude Area Defense units to South Korea on Mar. 6. The units are designed to shoot down ballistic missiles like the ones recently launched by North Korea into the Sea of Japan. (Youtube/Osan AirBase)

Given how angry Beijing gets about THAAD, you may be forgiven for thinking that the U.S. missile system, deployed to South Korea, is primarily aimed at China. However, Washington and Seoul have justified the system by saying it is necessary to defend South Korea from North Korean aggression.

With the missile system finally being deployed and tensions between the Koreas and China exploding again, here's a guide to the controversy.

What exactly is THAAD?

The acronym stands for Terminal High Altitude Area Defense. The ground-based missile defense system, which first came into development after the Persian Gulf War, is designed to shoot down short-, medium- and intermediate-range ballistic missiles during the terminal phase (i.e., when they are coming down).

According to Lockheed Martin, the U.S. company that manufacturers the system, there are four stages to its operation. First, a radar system identifies the incoming threat; then the target is identified and engaged. An “interceptor” is fired from a truck-mounted launcher, which destroys the missile using kinetic energy. Because the incoming missile is destroyed at a high altitude, the effects of weapons of mass destruction can be mitigated with the device.

The system is designed to be highly mobile, and it consists of four main components: a truck-mounted launcher; eight interceptors on the launcher; a transportable radar system; and a fire control system that links the various components with external command centers.

So why is it in the news right now?

THAAD systems have been deployed in a number of places around the world, including Guam and Hawaii. However, last year the Defense Department announced it would deploy a system to South Korea, where it would be operated by U.S. forces stationed in the country. In a statement, the Pentagon described the move as a “defensive measure” against North Korea after the country continued to pursue nuclear weapons and tested a number of ballistic missile systems.

This week, the situation on the Korean Peninsula escalated dramatically. On Tuesday, North Korean state media reported that the country has practiced attempts to hit U.S. military bases in Japan with a number of recently launched missiles. The number of missiles fired suggested that North Korea was training to see how quickly it could set up its extended-range missiles in a wartime setting.

That same day, the United States announced that it has begun deploying THAAD to South Korea this week. While the land where the system is due to be deployed is not ready yet, the equipment will be kept at a U.S. air base in Osan until the site is prepared. Full deployment is expected as early as June.

The news of the THAAD deployment sparked a threat of “consequences” from China. “I want to emphasize that we firmly oppose the deployment of THAAD,” said Geng Shuang, a spokesman for China’s Foreign Ministry, at a daily news briefing in Beijing on Tuesday. “We will resolutely take necessary measures to defend our security interests.”

Why is China so angry?

On the surface of it, it may be hard to understand Beijing's fury with THAAD. For one thing, it's a purely defensive system — THAAD systems don't carry warheads, relying on the force of the “interceptor” to destroy the incoming missile rather than a detonation. And while, in theory, the system could be used to intercept Chinese ballistic missiles, it would only work on missiles in their terminal phase: ruling out those targeting the United States, which would still be ascending.

Moreover, while China is well-known as a key ally and “big brother” of North Korea, it has shown itself to be exasperated by Pyongyang's recent missile launches. Beijing recently blocked coal imports from North Korea, striking a major blow to the isolated nation's economy.

Instead, many experts argue that China's anger over THAAD has less to do with the missiles than with the sophisticated radar capabilities included in the system. These radars could be used to track China's own missile systems, potentially giving the United States a major advantage in any future conflict with China. Some Chinese analysts argue that THAAD itself is of only limited use against North Korea anyway, as it would not be able to take out short-range missiles and artillery that do not reach high altitudes, hinting that the radar may be the real reason for the deployment.

More broadly, Beijing is concerned that the United States is hoping to use both South Korea and Japan to contain China in the future. “If South Korea insists on becoming a US puppet, China will have to act against it,” the nationalist state newspaper Global Times wrote in an editorial early this year.

How is Beijing retaliating?

Chinese reaction appears to be moving to hurt South Korea economically. Beijing has already placed restrictions on Korean businesses that operate in China, including shutting down stores of Lotte, a South Korean conglomerate that offered a golf course as land for THAAD use.

More measures are expected. Chinese travel agencies are stopping the sale of tickets to South Korea, and there have been growing calls in China to boycott South Korean products and even cancel tours by K-pop stars. Such moves carry significant weight. South Korea has grown increasingly dependent economically on China in recent years. China is South Korea's largest trading partner, and the value of its exports to the country were $142 billion in 2014 — more than twice the value of its exports to the United States.

Chinese pressure also comes at a time of political transition in South Korea. Scandal has sidelined President Park Geun-hye, who is expected to find out this week whether she will be impeached. And Moon Jae-in, the leading liberal candidate to replace her, has suggested he may reconsider the THAAD deployment.

What does this mean for the United States?

During last year's presidential campaign, Donald Trump spoke frequently of the threat posed by North Korea and suggested he may lean on China, which he said had “total control over North Korea.” However, since taking office in January, the new U.S. president has been uncharacteristically mute on the subject, despite provocations from Pyongyang. This has left many North Korea-watchers wondering what exactly his eventual policy will be.

The deployment of THAAD seems to be one of the first real moves against North Korea, though it was largely a continuation of policies undertaken by President Barack Obama. The backlash from China and various dramatic moves by North Korea, however, show that the situation is complicated.

Read more:

China warns of ‘consequences’ over deployment of U.S. antimissile system