the manufacturer of the Aegis system on Guam. That’s something akin to what Israel fields, a four-tiered system often lumped under the “Iron Dome” moniker, after its best-known and lowest layer.

The U.S. Army is testing the missile-defense system known as the Iron Dome at a military base in Guam amid concerns from Washington about China’s rapid development of missiles. Image: Adam Falk/WSJ ...