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Rather than read endless briefing papers, US and British generals could do worse than to watch Go Tell the Spartans, a little known but rather good film starring Burt Lancaster, about a US military adviser team in South Vietnam. In 1964, thousands of US advisers were present with the South Vietamese forces—and as those forces crumbled, more and more US advisers were being killed, sometimes by Vietcong agents among the US allies. Sooner or later, that is going to compel a choice: to pull the advisers out and see your client state collapse; or leave them there and watch them being slaughtered, which is politically impossible; or to send in a massive army of your own, which is what the US did— disastrously—in Vietnam. I don’t think that any responsible officer would wish to put...