After returning to the U.S., Hammer entered into a diverse array of business, art, cultural, and humanitarian endeavors, including investing in various U.S. oil production efforts. These oil investments were later parlayed into control of Occidental Petroleum. National Geographic described Occidental chairman Hammer as "a pioneer in the synfuels boom." Throughout his life he continued personal and business dealings with the Soviet Union, despite the Cold War. In later years he lobbied and traveled extensively at a great personal expense, working for peace between the United States and the Communist countries of the world, including ferrying physicians and supplies into the Soviet Union to help Chernobyl survivors. In his book "The Prize", Daniel Yergin writes that Hammer "ended up as a go-between for five Soviet General Secretaries and seven U.S. Presidents."