How should the U.S. respond to the massive Russia-linked hack?

Hackers believed to be connected to the Russian government breached the computer networks of both the private sector and some of the top agencies in the United States government, including the Department of Defense, the State Department, the Department of Homeland Security, the Treasury Department and the Commerce Department.

The full extent of the hack, which began as early as March, is unknown. But it is already considered one of the most expansive cyber intrusions in U.S. history. It’s also unclear whether the hackers merely gained access to information on...