This story was published in partnership with The Center for Public Integrity. This is the seventh in a 10-part series on nuclear risk, military technology and the future of warfare in light of Russia's invasion of Ukraine.

Russia's war on Ukraine has brought renewed attention to tactical nuclear weapons, with the White House announcing on March 24 that it had a team of experts briefing NATO on contingency plans in case any such weapon is used. 

As a defensive alliance, NATO has formally stayed out of the war, though many member countries have sent military supplies to Ukraine, from tiny Luxembourg to the United States.