Scenario for a new Korean War

If North Korea decides to invade southward, it will probably come after China begins its war with Taiwan. China and North Korea are de facto allies and so both benefit from having another conflict to act as a second front for their enemies. South Korea and the US have already depleted a large portion of their artillery shells by sending it as military aid to help Ukraine. North Korea, on the other hand, while it has sold some of its munitions to Russia, it has also earned ample cash and Russian technology to upgrade its industry and weaponry. 

There will not be a long delay between the outbreak of a war between China and Taiwan and a new Korean War. The US will be forced to make hasty decisions about which of its allies to aid first and how much. South Korea would likely get priority as there are many US troops stationed there, unlike Taiwan which hosts none. China has a significant airpower and missile advantage over Taiwan, and many US air defense systems like the Patriot currently tied up in Ukraine. 

If North Korea attacks, it will likely kill the US troops mingled with the South Korean military. In that case, they might as well launch a non-nuclear ballistic missile attack on Hawaii because the political and propaganda consequences are the same. In a similar way, if China destroys US aircraft or ships caught in the crossfire with Taiwan, they might bombard US bases in Guam and elsewhere. 

The basic Chinese strategy would be to win a quick victory over Taiwan before US and other aid can arrive. Meanwhile, the North Korean strategy revolves around getting Chinese support long-term in exchange for acting as a diversion for US forces. 

The US has never gone to war with a nuclear power, much less two simultaneously. Both China and North Korea would use nuclear weapons if the survival of their respective regimes was at stake. Recent US history has shown its public has a much lower tolerance for casualties than other countries. The US military continues to struggle with recruitment problems, and a new large-scale war would only exacerbate that. 

The best way for the US and its allies to guarantee the sovereignty of Taiwan and South Korea would be to prevent a war with a Chinese-North Korean alliance. Such a war could only be won at an enormous cost, if it could be even won at all. The single most important factor for preventing such a war would be to bring the war in Ukraine to an end as soon as possible. The longer it continues, the greater the odds that there will be a new Korean War with the South China Sea as a secondary theater. 

If the status quo in Ukraine continues, it will be necessary to use other forms of deterrence. The US Navy has the best capabilities for that at present. The bulk of the Navy's fleets should be stationed in the Pacific at its ports in Japan and nearby. Subic Bay, Yokosuka, and Busan are preferable to Hawaii and Guam for that purpose. It's possible that such a deployment combined with large-scale military exercises could have the side effect of forcing the war in Ukraine to end.  It could also reduce tensions in the Middle East.