You need good reflexes to write about Donald Trump. One day he’s raising tariffs to levels that might crash the world economy. The next he’s poised to sign dozens of trade deals, many of which are likely to anger China. Earlier this month his administration cut off arms shipments to Ukraine; yesterday it abruptly decided not to. Presidential zig-zagging would be thrilling entertainment if the stakes weren’t so high. Guessing the next move is tricky, but there’s plenty to say about the consequences of what he’s already done.