Russia's Vladimir Putin threatened a nuclear strike on Poland. He suspended all co-operation with the US alliance bloc in Europe, NATO. And he set in train the events which culminated in the annexation of Crimea, the invasion of Ukraine and the shooting down of MH17 by Russian-backed Ukrainian rebels. Why is it so momentous to install a missile defence system? Because it will defang the Russian bear. It will make Russian missiles a less potent threat and upset the balance of mutually assured destruction in the event of a nuclear war. It will embolden the East Europeans to take a harder line in dealings with Russia. And it will diminish Russia as a danger. Which is why Putin resorted to other measures to make Russia more dangerous again. In the case of South Korea, Beijing has warned it publicly, repeatedly, against doing any such thing. But now North Korea has pushed Seoul too far. The UN Security Council has explicitly forbidden Pyongyang from testing ballistic missiles, so the North Koreans claimed their weekend launch was the harmless deployment of a satellite. Absolutely no one buys this. Provoked one time too many, Washington and Seoul announced they would seek to deploy a Terminal High-Altitude Air Defence system "at the earliest possible date". The system is to be operated by US forces.

China will be upset "because it believes that the system is not designed to stop North Korean weapons but Chinese weapons", says a professor of international security at the University of NSW, Alan Dupont​. "There's some substance to the Chinese concern," says Dupont, "because THAAD could be used to partially nullify Chinese ballistic missiles." It was, nonetheless, the right decision for South Korea's President Park Geun-Hye "South Korea has no alternative given the domestic political pressure. The North Korean capability is steadily improving and THAAD is the only game in town" for countering it, says Dupont. "The Chinese will be pissed off at the North Koreans." Indeed. Their supposed friend and ally in Pyongyang, the baby-faced Kim Jong-un, has, in effect, invited the US to bring a missile defence shield onto the Asian mainland for the first time. The US has already deployed a missile defence system in Japan at Tokyo's request. And it has installed another on the American Pacific island of Guam, a major US base. Now that the US has been invited to deploy the system in Seoul as well, it will not only diminish the potency of any Chinese missile threat to South Korea. It will also provoke other countries in the region to consider doing the same, Australia among them.

Four lessons stand out. First, it's an object lesson for China in why it should restrain its rogue ally. Out of its own self-interest, China should have done more to check North Korea. By allowing Pyongyang to enlarge itself as a danger to its neighbours, China has given the US a strategic opportunity to improve its relations with Seoul. South Korea is a major power of North-East Asia and a country that has been showing signs of moving out of the US sphere of influence and into the Chinese. This helps cement it into the US sphere. Second, it is an object lesson to China about why it should be more restrained itself. The Korean episode is an example of what happens to regional nations who feel their security to be under threat – they turn to the US for protection. The more the Chinese push other countries around with their aggressive island-building and territory-grabbing tactics in the South China Sea, the more incentive those countries will have to seek American protection. Third, it's a reminder to other countries in the Asia-Pacific region that the US may be a diminished superpower, but it's still a superpower. It can still be a uniquely valuable partner to countries in fear of their neighbours. Fourth, as countries weigh their future security choices between China and America, a defining difference is that China is a country with some pretty unpalatable allies, whereas the US is at the centre of an entire system of alliances. It's a system much bigger than the sum of its parts. And, as Australia takes delivery of its air warfare destroyer ships in the next few years, Canberra will have the option of doing what the South Koreans are now doing.

The Aegis missile systems on the Australian ships will allow Canberra to put Australia into a US missile defence system too. As North Korea's missiles, already capable of reaching Singapore, extend their range, it's an idea that will be increasingly attractive to any Australian government. Peter Hartcher is the international editor.