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Other initiatives

Polk's ambassador to the Republic of New Granada, Benjamin Alden Bidlack, negotiated the Mallarino–Bidlack Treaty. Though Washington had initially only sought to remove tariffs on American goods, Bidlack and New Granadan's Foreign Minister negotiated a broad agreement that deepened military and trade ties. A U.S. guarantee of New Granada's sovereignty over the Isthmus of Panama was also included. The treaty was ratified in 1848 and in the long run it facilitated the Panama Canal, built in the early 20th century. It also allowed for the construction of the Panama Railway, which opened in 1855. The railway, built and operated by Americans and protected by the U.S. military, gave a quicker, safer journey to California and Oregon. The agreement was the only alliance Washington made in the 19th century. It established a strong American role in Central America and was a counterweight to British influence there.

In mid-1848, President Polk authorized his ambassador to Spain, Romulus Mitchell Saunders, to negotiate the purchase of Cuba and offer Spain up to $100 million, a large sum at the time for one territory, equal to $ 3.52 billion in present-day terms. Cuba was close to the United States and had slavery, so the idea appealed to Southerners but was unwelcome in the North. However, Spain was still making profits in Cuba (notably in sugar, molasses, rum and tobacco), and thus the Spanish government rejected Saunders's overtures. Though Polk was eager to acquire Cuba, he refused to support the filibuster expedition of Narciso López, who sought to invade and take over the island as a prelude to annexation.