The writer is a professor at Johns Hopkins University and co-author of ‘Underground Empire: How America Weaponized the World Economy’ with Abraham Newman, who also contributed

Donald Trump likes to say that he alone can protect America from being “ripped off” by greedy allies. So why does he want to replace the one cornerstone of national security that foreigners subsidise, with a system that would make American consumers pay the costs? 

In recent weeks, Trump has talked about moving away from US financial sanctions against Russia and China, which he claims are undermining the dollar and making China’s currency more attractive. Instead Trump wants to turn tariffs into America’s go-to tool for coercion. The threat of 100 per cent tariffs might force reluctant governments to stick with the dollar, or coerce skinflint Nato members into spending more on their military.