I don't remember that the reason for going into Vietnam or Lebanon was even partially about regime change. If in 20 years you see that Iraq continues in some form of (mostly) secular democracy that will be seen as a success.
Vietnam was a cluster from start to finish and not really our war to begin with. Lebanon was an attempt to negotiate peace in an area where we just stuck our foot into the water, and realized we really didn't have any good reason or policy on which to continue moving forward.
How Lebanon tarnishes Reagan Legacy:
http://www.reason.com/news/show/33611.html
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The Reagan administration did not initially go into Lebanon to engage in nation-building. Indeed, soon after peace returned in September 1982, the MNF departed from the country earlier than scheduled. Geoffrey Kemp of the Nixon Center, who was on the National Security Council at the time, has noted that the only thing that brought them back was the massacre of Palestinian civilians at the Sabra and Shatila refugee camps in Beirut following the entry of Israeli forces into the Lebanese capital. This was prompted by the assassination of Lebanon's president-elect, Bashir Gemayel.
Subsequently, the administration was divided over how to proceed. Secretary of State George Shultz worked hard to bring about a Lebanese-Israeli withdrawal agreement. Reagan went along with this for a time, until the effort collapsed in 1983. Secretary of Defense Caspar Weinberger, meanwhile, wanted to exit from Lebanon as quickly as possible. Either way, there was no solid administration constituency for remaining in the country after American political and military setbacks increased. That's why it was easy for Reagan to change tack in 1984 once Shultz realized that his scheme had failed.
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