When Shiva Chidambaram started putting money into his 401(k) a couple of years ago, he didn’t think much about his investments. Then the war in the Middle East began.

Chidambaram came across a tweet about U.S. weapons manufacturers last month that prompted him to investigate the stocks in his retirement account. He discovered his 401(k) contributions were held in a target-date fund that owns shares of military contractors such as Lockheed Martin and Northrop Grumman.