Lt. Gen. Robert Rasch, the joint program officer for the Guam Defense System, testified at a Senate hearing this week that the cost of providing an integrated air and missile defense system — in effect a miniature “Golden Dome,” for Guam — would total approximately $8 billion. These funds would cover the cost of placing sensors and launchers, as well as the command-and-control systems that would link them.

All these systems either already exist or are in later stages of development; no new development would be involved.

Responding to Rasch’s cost estimate, Sen. Angus King (I-Maine) extrapolated the $8 billion figure to account for the 779 cities in the U.S. that are equal to or greater than Guam’s population. According to his calculation, a Golden Dome over all of America would total $6.2 trillion.

And King’s figure was a bit too low; applying the same methodology results in a total cost of $6.4 trillion.

Whether that $6 trillion figure is even remotely accurate is far from clear. Extrapolating the cost of defending Guam to the continental U.S., but calculating not on the basis of the number of cities to be defended but rather on the island’s area in square miles relative to the mainland, yields a ratio of........

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