A strongman president, self-styled redeemer of national glory, is trapped in a conflict he can’t win but doesn’t know how to end without looking like a loser. A cult of infallibility prevents the leader admitting a strategic blunder even to himself. It could be Donald Trump or Vladimir Putin; Iran or Ukraine.The conflicts and the regimes involved are also dissimilar in important ways. Russia’s campaign to eradicate a neighbouring democracy is nastier in conception and bloodier in execution than the bungled US effort to dislodge a dictatorship in Tehran. It has also gone on much longer. The first world war...