On Wednesday, Colombia, the United States and nine other countries invoked the Inter-American Treaty of Reciprocal Assistance (TIAR), signed in Rio de Janeiro in 1947, which commits the countries of the Western Hemisphere to respond to military aggression against any one of them. The move came after Nicolás Maduro said he would deploy 150,000 troops to the border with Colombia. Invoking TIAR is an extreme measure in the region and an unmistakable sign that armed conflict is now a real possibility.