I may not be correct but I believe that Martin Luther King implied that if they were to protest with violent it would only result in more violence as a retaliation by the government. So by doing peaceful protest the government didnt have an actual solid reason to attack them, so when the government did enact violence on them it caught the attention of people around the world.
The U.S. government openly stated that they would view any European interference in Latin America as "an act of aggression". This was articulated clearly in the Monroe Doctrine, which stated that the US would respond with military force.
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