As its central bank, the Fed, winds down its quantitative easing program by selling off its stock of accumulated assets to the private sector, investors need dollars to buy them. This causes the number of dollars available to investors to shrink by up to $50bn a month. Add to this the dollars German and Chinese capitalists need to buy US government bonds (in a bid to park their profits somewhere safe) and you begin to see why Trump believes he will not be punished by a run either on the dollar or on government bonds.
Armed with the exorbitant privilege that owning the dollar presses affords him, Trump then takes a look at the trade flows with the rest of the G7 and comes to an inescapable conclusion: he cannot possibly lose a trade war against countries that have such high surpluses with the US (eg Germany, Italy, China), or which (like Canada) will catch pneumonia the moment the American economy catches the common cold.
Yanis nailed it!