President Donald Trump’s tweet last weekend that the US might terminate all tradewith countries doing business with North Korea was widely derided on the grounds of realism. Given that 90 percent of North Korea’s trade is with China, the tweet was little more than a veiled threat to terminate all US trade with Beijing, ending a bilateral trade relationship valued at $650 billion a year. It would, as many correctly pointed out, mean economic disaster for North Korea -- and also for the US. The “realism” argument -- as well as its companion criticism that such statements call into question US credibility -- is well founded. But my bigger issue with the president’s tweet lies elsewhere. Trumps threat -- particularly when viewed alongside his reported preparation to withdraw from the free trade agreement with South Korea -- shows that he is willing and inclined to take on would-be allies in the hopes that the pressure he creates will force them to address a common problem on his terms. By threatening to curtail US trade with South Korea and China, Trump seems to be trying to create some leverage, some bargaining power, over Beijing and Seoul that he can then use to get them to be tougher on North Korea. As he said back in April on "Face the Nation": “I think that, frankly, North Korea is maybe more important than trade.” He also mused that securing China’s help in dealing with North Korea could be “worth making not as good a trade deal for the United States.” We have known for some time that the president views all negotiations through such a transactional lens. And perhaps, in some circumstance, such a transactional approach can bear fruit in the realm of foreign policy, although I am hard pressed to point to one. Some US administrations have tried to deal with Russia in this way; Moscow generally saw any concession as a sign of weakness, rather than as part of an explicit bargain. Taking this approach with allies such as South Korea or potential partners such as China (at least in regard to North Korea) is even more problematic. In these instances, the possibilities should be greater, based on the assumption that the relationship is bigger than a cost-benefit analysis. The arrangements that make sense in a context of some trust may otherwise fall flat in a context only of short-term gain. But what is so surprising and unsettling about the president’s statements of the last couple of days -- particularly in the aftermath of North Korea’s most powerful nuclear test yet -- is that Trump seems to think this transactional approach is appropriate even in a crisis that could be existential to some of the parties. Trump appears to believe he can tweak the balance sheet of these relationships in a way that both China and South Korea see no choice but to approach the North Korea problems as he would like them to do. This is highly risky. Finding a solution to -- or, more likely, the sustainable management of -- a problem as high-stakes and complex as North Korea demands an entirely different approach. In the past, when collective action was required to combat a looming threat, each actor was often required to take what was sometimes an enormous leap of faith, and commit to pledges and take risks dramatically outside its comfort zone. Such leaps were possible in the context of trust and common values and purpose -- such as NATO members uniting behind the idea that an attack against one is an attack against all. Sometimes this coming together was less noble and unconditional -- such as when the US and China started their rapprochement in the 1970s. But this was done against the backdrop of perceived and real common interests in an alignment against the Soviet Union, and was broad-based enough to survive some disappointments on both sides. It was bigger than the outgrowth of a series of short-term quantifiable calculations, weighed against one another on an Excel spreadsheet. I have real sympathy for those in the Trump administration charged with addressing the threat from North Korea. It is a policy problem of enormous proportions, with no obvious right answer. There are, however, obviously wrong strategies, and adopting a hard-core transactional approach toward allies and necessary partners is among them. To the extent it is possible to imagine a positive outcome to this crisis, it involves the US, China, South Korea and others working together -- and almost certainly putting some element of trust and confidence in each other to uphold promises and commitments that cannot be externally guaranteed. A transactional approach stands no chance of getting us that far. Bloomberg View