Plausibility of Modern or Future Filibuster Raids?

No, not the Jimmy Stewart kind of filibuster. The William Walker variety is what I have in mind. What would it take for random groups of American adventurers or mercenaries to try to conquer a poor developing country in the 20th or 21st centuries without being backed or endorsed by the U.S. government? Why was such activity much more common in the 19th century? Can modern governments, even weak ones, nullify the filibuster threat?

Note that I never said that these filibusters have to succeed, given how William Walker and others of his ilk fared.

Please tell me if there were such people during this time period that I don't know about.
 
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