The Vietnam War is still argued over as either unnecessary or strategically successful, often using the very same outcomes as evidence. That shows why hindsight cannot settle the issue. Strategy has to be judged ex ante, by what leaders knew, feared, and believed at the time. Between 1955 and 1965, U.S. leaders saw Vietnam not in isolation but as part of a chain leading from Indochina to ASEAN, Indonesia, Japan, and ultimately the global order. My new essay sets out eight principles for analysing strategy in this way, using Vietnam as a case study with lessons that remain relevant today.