This one ranks up there with the best of them, a virtuoso performance of Nixonian foreign policy.  It's illegal enough to intervene in a country in order to establish democracy and free elections, but it reaches high comedy for America to intervene to overturn a free election and install a military dictatorship.[2]

Ever since the United States lost Cuba to socialism, it vigilantly guarded against a repeat of this miscalculation.  The U.S. played a large role in Chilean politics due to its copper mines, which were very important to, among others, ITT.  Salvador Allende, an aspiring socialist politician, had run for president in 1964 only lose out to Eduardo Frei, the candidate backed by the U.S. government.  Much to the chagrin of the U.S., and despite considerable CIA efforts to destabilize the economy, Allende won the presidential election of 1970.  This contravened the American doctrine that socialists could never win a free election.  Allende went about nationalizing the precious copper mines and implementing a follied statist program, and the U.S. continued working to cripple Chile financially.  However, the U.S. did funnel supplies to the Chilean military.  Unsurprisingly, the military eventually overthrew the Allende government and ensconced itself as the active governing authority for the next generation.  Pinochet, the military leader, although hardly the bloodiest dictator of all time, set about brutally quashing dissent.

Allende was definitely a lousy president, and some say that he was oppressive in his own right.  That may be true, but it is irrelevant to the legality of the United States' efforts to oust him.  International law does not mandate that only some forms of government are legal.  As long as the government possesses effective control over its national territory, it is the legitimate authority.  Even if we make the bold claim that democracy is the only legitimate form of government, how can we justify overturning an untainted election?

CIA Factbook on Chile

Libertarian view of Pinochet's abduction (Cato Institute)

Liberal view of Pinochet's abduction (Progressive Response)