After Obama’s trip to China, he became the first sitting U.S. president to visit Laos, where he has pledged $90 million to help clear Laos of unexploded U.S. bombs left from the United States’ secret bombing campaign in Laos during the Vietnam War. Between 1964 and 1973, the U.S. dropped as many as 270 million cluster bombs on Laos. Laos authorities say as many as one-third of these cluster bombs did not explode at the time.