The current issue of Missouri Conservationist mentions the change in Smokey Bear's message, changing his traditional "Forest Fires" message to "Remember...Only You Can Prevent Wildfires". The reason for the change is the increasing recognition of the importance of prescribed fire for the management of woodlands, prairies and glades. The Conservationist article, Prescribed Fire: A Management Tool, is a good overview on its uses and techniques.
Smokey was created as part of a WWII "homeland security" effort. Early in 1942, the Japanese made multiple attempts to set fires in the Oregon forests using float planes with incendiary bombs launched from submarines. In 1944 they released up to 9,000 fire balloons into the newly discovered jet stream with around 10% reaching the US. There was no significant fire damage although a teacher and 5 children died when they found a device and were tampering with it, the only war casualties to occur on US soil. (Wikipedia)
With the lack of available manpower to fight fires due to the war effort, there was a push for increased public awareness of forest fire dangers. The Forest Service had Smokey Bear created for the purpose, with the first poster above. Smokey the Bear came along a few years later in a popular song, "the" being added for rhythm purposes. The "e" in Smokey was a deliberate addition. The "the" has never been in his official name although the Forest Service have never been able to erase it from the mind of the public.
The lesson today is that fire is a legitimate tool in forest management but must be used with care. The article in Missouri Conservationist emphasizes safety through prescribed fire training which is available through MDC.