In searching for these data I came across another question on the same general issue that has been asked as far back as 1946, originally by Gallup and later by several other organizations: "Do you think it would be best for the future of this country if we take an active part in world affairs, or if we stay out of world affairs?" Answers are summarized in the following figure:
The basic pattern seems to be that support for an active role was higher in the 1950s and 1960s and has been at a lower level since then. It has bounced back a few times: in 1991, when the Soviet Union was starting to break up, and for several years after 9/11. The Pew question suggests more of a real trend away from support for international involvement in the last 10 year or so--this question has ups and downs without much pattern. It seems to me that the questions are asking pretty much the same thing, but maybe people interpret them differently. Or maybe it's related to the exact times that each question was asked.
[Note: data from iPOLL, Roper Center for Public Opinion Research]