The United States military has announced that the THAAD anti-ballistic missile defense system in South Korea — designed to intercept incoming missiles from North Korea — is now officially operational.

“U.S. Forces Korea confirms the Terminal High Altitude Area Defense (THAAD) system is operational and has the ability to intercept North Korean missiles and defend the Republic of the Korea,” said Col. Rob Manning, US Forces Korea spokesperson in a statement.

And while the North Koreans are certainly not thrilled about that, they’re far from the only ones upset about THAAD.

China adamantly opposes the deployment of THAAD, seeing the system as a threat to its military capabilities. "China calls for an immediate stop to the THAAD deployment on the Korean Peninsula," China's foreign ministry spokesperson Geng Shuang said during a regular press briefing on Tuesday.

"China will pursue protecting its interests going forward by taking the necessary measures in a stern manner," said Shuang.

The missile system is also facing criticism from some South Koreans worried about Chinese economic retaliation as well as some environmental and safety concerns. And now that South Korea’s pro-THAAD president has been impeached, THAAD’s future could be in jeopardy.

Here, then, is a quick guide to what THAAD does, and why so many people are so upset over it.

So what is THAAD, exactly, and why is China so mad about it?

THAAD stands for Terminal High Altitude Area Defense. It’s designed to detect and then intercept incoming ballistic missiles in their “terminal” phase — that is, when they’re on the way down, not on the way up.

It’s a system that’s already deployed in Guam, and it’s now being deployed in South Korea to protect against any incoming missiles from the North. According to data from the US Missile Defense Agency, which conducts regular tests of US ballistic missile defense systems, the THAAD system has had 13 successful intercepts out of 13 attempts stretching back to 2006.

The THAAD system uses sophisticated radar to detect incoming missiles. In order to detect incoming missiles from North Korea, THAAD’s radar system needs to be pointed at — you guessed it — North Korea.

But it just so happens that when you point this sophisticated radar system at North Korea, you don’t just see North Korea — you also see parts of China. China and North Korea do share a border, after all, and the site from which these latest North Korean missiles were fired is really close to that border.

That means China is worried that THAAD’s radar system could potentially help the US better detect Chinese missiles being launched at the United States in the event of a future war. To the US military, of course, the ability to detect a Chinese nuke heading our way even earlier than we currently can sounds pretty great.

But for the Chinese military, this means the US now has a slight edge. The Chinese military can’t put radar anywhere near that close to the US to detect incoming US nukes, which means that in a nuclear conflict, the US would have a slight strategic advantage — it would be able to detect an incoming Chinese nuke and respond faster than China could in the reverse.

Getting super worked up about another country potentially having a very slight strategic advantage over you in some hypothetical future war might seem a tad ridiculous to regular folks like you and me — and especially to those who didn’t live through the darkest days of the Cold War, when such matters felt much more real and immediate — but these sorts of things are taken very seriously by militaries, and China is no exception.

“I want to emphasize again that China is firm in its resolve to oppose the deployment of THAAD in the ROK [Republic of Korea] and will resolutely take necessary actions to safeguard its own security interests,” said Chinese Foreign Ministry spokesperson Shuang at a press briefing in Beijing in late February.

“Any consequences entailed from that will be borne by the US and the ROK. We strongly urge relevant parties to stop the deployment process and not to go further down that wrong path,” he said.

Some people in South Korea aren’t thrilled about THAAD, either

The specter of Chinese economic retaliation against South Korea is one of the main reasons why some in South Korea oppose THAAD.

And indeed, China has already begun to impose some of those “consequences” on South Korea — or, more specifically, on one of South Korea’s biggest companies. The company in question is the Lotte Group, a multinational conglomerate headquartered in Seoul. Last week, it agreed to give up a parcel of land it owned to the South Korean government to use as a base for the THAAD system.

Then, all of a sudden, at least 23 Lotte Mart stores across China were mysteriously shut down by Chinese authorities. As CNN reports, Chinese officials claim they were shut down over violations of fire safety regulations, but the timing is a bit suspicious, to say the least.

And, as my colleague Lindsay Maizland has written, “Korean TV shows and K-pop music videos have been blocked from streaming in China — one of their biggest and most lucrative markets — Chinese internet users have posted about boycotting Korean beauty products, and Korean celebrities have canceled tours in China.”

But fear of economic retaliation isn’t the only reason some South Koreans oppose THAAD. There are also safety and environmental concerns among local residents in the area where THAAD is being deployed.

Back in August, about 900 South Koreans shaved their heads in a mass demonstration against the government’s decision to house THAAD in the southeastern county of Seongju, a region famed for its melon farming. Activists said they were concerned that the system's sophisticated radar could harm their crops and that having a missile system nearby would potentially make the area a target in wartime.

At another demonstration in July, the governor of Seongju stood in front of a crowd of 5,000 protesters and wrote “No to the deployment of THAAD in Seongju” using his own blood.

The New York Times reports that some critics in South Korea are also upset with the government’s choice of Seongju as the THAAD site because putting it there will mean that the country’s capital, Seoul, will be outside the coverage of THAAD’s intercept missiles.

And now that South Korea’s President Park Geun-hye, who was a staunch supporter of THAAD, has been impeached and removed from office, THAAD’s future could be in some jeopardy. Snap elections to replace Park are scheduled for May 9 — just a few days from now — and the leading candidate, Moon Jae-in of the liberal Democratic Party, has called for halting the deployment of THAAD “until the new president takes office and can evaluate its benefits and drawbacks.”

“The delivery should be halted even how, and the next administration should ultimately decide this issue,” Moon Jae-in’s campaign manager said Wednesday.

“The next administration, however, will continue to face an excruciating dilemma,” writes South Korea expert Benjamin Lee in the Diplomat. “If South Korea decides to revoke the THAAD decision, this will set a terrible precedent, which will cause China to believe that it can use its economic influence over South Korea to control Seoul’s strategic agenda.”

For now, at least, THAAD’s deployment to South Korea looks like a done deal.