What is the difference in meanings and nuances between "capacity" and "ability" as in the following newspaper article in The Washington Post of today.
"Defense Secretary Chuck Hagel on Wednesday called the North Korean actions a “real and clear danger and threat” to the United States’ allies in the region, South Korea and Japan. “They have nuclear capacity now. They have missile delivery capacity now.”
…The Pentagon declined to specify where the systems would come from. “Though we do have a limited number of THAAD units available for deployment, we are quite confident in our ability to rapidly redeploy this system as dictated by threat levels,” said Defense Department spokeswoman Lt. Col. Monica Matoush."