U.S. and Britain Present a United Front

President Obama said Friday that the United States and Britain have pledged new cooperation to bolster cybersecurity and an intensified effort against the Islamic State as the two nations work to confront a mounting threat from violent extremists across the globe in the wake of terrorist attacks in Paris.

“This phenomenon of violent extremism — the ideology, the networks, the capacity to recruit young people — this has metastasized and it is widespread, and it has penetrated communities around the world,” Mr. Obama said at a news conference in the...