Mexico promptly severed diplomatic relations with the United States. As Mexico had never recognized the independence of Texas, she had of course never agreed upon any boundary with the new republic. This was a matter which had to be adjusted and there were also a number of private claims of American citizens against the government of Mexico which that government refused to settle. President Polk took up both questions with characteristic vigor, and on the refusal of Mexico to receive a special minister sent by him for the purpose of discussing these questions, he ordered General Taylor to occupy the disputed area between the Nueces river and the Rio Grande. Thus began the Mexican War, which established the boundary of the United States on the Rio Grande and added the vast region of New Mexico and California to the Union. Here the tide of American expansion to the South was stayed for a full half century. With the decline of the Spanish power Great Britain had succeeded to naval supremacy in the Caribbean. As has been related in previous chapters, the United States and Great Britain long regarded Cuba with jealous eyes and had a controversy lasting for half a century over the control of the proposed Isthmian canal. Secretary Seward at the close of the civil war sought to strengthen the position of the United States in the Caribbean by the acquisition of Santo Domingo and the Danish West Indies. In 1867 a treaty was concluded with Denmark providing for the cession of the islands of St. Thomas and St. John for $7,500,000, on condition that the inhabitants should by popular vote give their consent. In undertaking these negotiations the United States was influenced on the one hand by the desire to acquire a naval base, and on the other by the fear that these islands might fall into the hands of one of the greater European powers. The plebiscite in St. John and St. Thomas was overwhelmingly in favor of the cession, and the treaty was promptly ratified by the Danish Rigsdag, but the Senate of the United States took no action until March, 1870, when Senator Sumner presented an adverse report from the Committee on Foreign Relations and the treaty was rejected.