Countries shore up their digital defenses as global tensions raise the threat of cyberwarfare

Hackers linked to Russia’s government launched a cyberattack last spring against municipal water plants in rural Texas. At one plant in Muleshoe, population 5,000, water began to overflow. Officials had to unplug the system and run the plant manually.

The hackers weren’t trying to taint the water supply. They didn’t ask for a ransom. Authorities determined the intrusion was designed to test the vulnerabilities of America’s public infrastructure. It was also a warning: In the 21st century, it takes more than oceans and an army to keep the United…