Accompanied by British director Jeremy Gilley and a film crew, the 34-year-old actor interviewed children, government ministers and community leaders for a movie to mark the U.N.'s Peace Day on Sept. 21.
The United Nations General Assembly adopted Peace Day into its global calendar of events in 2001, following Gilley's lobbying campaign, which he documented in the film "Peace One Day."
"Afghanistan will be the focal point for the second film, so it seemed important that Jeremy also have a sounding board, a third-party perspective on his trip," Law told reporters Thursday inside a guarded U.N. compound in Kabul.
"This film is about documenting and seeing how Peace Day can save lives," Law said.
Law's trip was conducted in secrecy for security reasons. The crew also filmed schools and visited United Nations Children's Fund-supported programs in Kabul and in the eastern provinces of Laghman and Nangarhar, where violence often flares.
"This is a country I'd always wanted to visit," Law said. "It was a situation that I found intriguing. I felt that if it was safe for Jeremy it must be safe for me."
Gilley said the film will be shown in 50 countries and up to 70 international film festivals.
"Whether there is fighting in our countries, whether there is fighting in our homes, in our communities or in our schools, the message is to us all, there is one day of peace on this planet and if peace is important ... then we will all do everything that we can on that day to send a message of hope to the people of our world," Gilley said.
Law received Oscar nominations for his roles in "The Talented Mr. Ripley" and "Cold Mountain."