LONDON  – Citing tensions with Iran and North Korea, Prime Minister David Cameron offered a strong defense of Britain's nuclear deterrent Thursday, traveling to Scotland to challenge nationalists seeking independence who want to expel nuclear-armed submarines from Scottish bases.

Blending geopolitics with his opposition to Scottish independence, Cameron welcomed home the crew of a nuclear-armed submarine based in Scotland and told defense industry workers that their jobs were more secure in a united Britain. Britain maintains a force of four nuclear submarines that patrol year-round carrying Trident missiles. But lawmakers are locked in a debate about the cost of replacing them at a time when Scottish nationalists, committed to nuclear disarmament, say they will not allow the submarines to be based in an independent Scotland.

In an article in the Daily Telegraph, Cameron said, "There are those who say that we don't need it anymore, because the Cold War has ended. ... In terms of uncertainty and potential risk it has, if anything, increased."

He described the North Korean government as "highly unpredictable and aggressive."

new york times