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A sovereign state is, by definition, supposed to manage affairs inside its borders. But that's not always the case, especially when it comes to disputes involving guerrilla movements. After all, moral equivalency or not, one nation's terrorists really are another's freedom fighters, and foreign governments sometimes cross international borders to protect antigovernment forces elsewhere, reinforce ethnic movements, or simply to make their presence known. Last week, for example, documents revealed that Venezuela is still supporting the FARC guerrillas in Colombia.

Not surprisingly, this uninvited meddling is not received warmly; intrusions often cause major international crises. By offering military aid, financial support, and political alliances to rebel groups, nations can sour...