[nickff]

Diego Garcia is not part of a defensive line, it is a force multiplier. The problem with using an island chain to defend yourself is that you must keep all the islands supplied, and this is a logistical nightmare. A single island is much easier to supply and defend.

Diego Garcia is a single island, more like Malta than the Japanese islands and islets. Diego Garcia's capacity for basing large strategic bombers close to potential conflict zones is a huge force multiplier, much like basing aircraft in Malta allowed the Allies to disrupt axis resupply efforts to North Africa.