Pop star Jessica Simpson has identified the precise event that led to her divorce from fellow singer Nick Lachey.

According the Simpson, she saw the end of their marriage in October 2005, when Lachey chose not to go with her on a charity tour to Africa.

"I went there on our three-year wedding anniversary," she says in an interview with Jane magazine, set to appear in their November issue. "He stayed home."

Simpson went on the trip as an ambassador with Operation Smile, an organization that provides volunteer medical services for children requiring reconstructive facial operations.

Both Simpson and Lachey had been given Universal Smile Awards by the charity, but only Simpson participated in the tour.

"On that day, everything became so clear," she says. "I was in hospitals with all these sick kids. I just knew I needed to find something more in my life, on my own."