When we last saw John Rambo — Vietnam vet, social pariah, savior of POWs and stoic killing machine — he was saving missionaries and mercenaries in war-torn Burma at the end of Bush II’s second term.

Rambo’s cartoonish action feats may have been riveting in the Reagan years, but these days, they’re just a tiresome slog. Midway through “Rambo: Last Blood,” Sylvester Stallone mumbles a heartfelt ...