Neither side has taken any further steps to avoid war. Meanwhile, Polk resolved a major territorial dispute with Great Britain over the Oregon Treaty, signed on June 15, 1846. By avoiding a chance of conflict with Britain, the United States has been given carte blanche to Mexico. After the Thornton case of April 25-26, when Mexican troops attacked an American unit in the disputed area, with the result that 11 Americans were killed, 5 wounded and 49 captured, Congress adopted a declaration of war signed by Polk on May 13, 1846. The Mexican Congress responded with its own declaration of war of July 7, 1846. [Quote needed] The treaty expands the choice of U.S. citizenship to Mexicans in newly acquired territories before many African-Americans, Asians and Indians are allowed. If they decided, they had to declare to the U.S. government, within one year, that the contract had been signed; Otherwise, they could remain Mexican citizens, but they would have to move. Between 1850 and 1920, the census counted most Mexicans as “white” racially. Yet racist tensions continued after the annexation, reflected in things like the Greaser Act in California, when suddenly tens of thousands of Mexican nationals found themselves within the borders of the United States. Mexican communities remained de facto separated from and within other American communities and continued in the southwest through Mexican migration until the end of the 20th century. [Citation required] Mexico had claimed the territory in question since its independence from the Spanish Empire in 1821, after the Mexican War of Independence. The Spanish had conquered part of the territory of the Indian tribes over the past three centuries, but they remained powerful and independent indigenous nations in this region of northern Mexico. Most of this country was too dry (little rainfall) and too mountainous to support many people until the birth of the new technology after about 1880: means of congestion and distribution of water from the few streams on irrigated arable land; The Telegraph; Railways; The phone and electric power. Despite the long series of military defeats, the Mexican government was reluctant to accept the loss of California and New Mexico. Even with its capital under hostile occupation, the Mexican government was inclined to take into account factors such as the reluctance of the U.S. government to directly annex Mexico and the apparently deep division of American domestic opinion on the war and its objectives, which led to the conclusion that it was in fact in a much better negotiating position than the military situation suggested. [Citation required] Another aspect was the Mexican government`s opposition to slavery and its awareness of the known and growing divide in the United States.