The United States had annexed Texas, which had recently gained its independence from Mexico, in 1845. The Mexican government, angry over this development, disputed the border of the Republic with the United States, which claimed the southern border of the new state extended south to the Rio Grande. Polk claimed that the Mexican government, which he characterized as a military dictatorship, would not receive the envoy, John Slidell, that he sent to discuss the situation. In addition to this, Polk's primary justification for war was that a Mexican force had attacked an American cavalry force on Texas soil (which, of course, the Mexican government recognized as their own territory.) He went on to claim that the Mexican government refused to conduct commerce and diplomacy and that it had assumed a generally warlike posture toward the United States. But the primary justification was the (highly disputed) attack on American troops on the Nueces River, which he characterized as Mexico having "invaded our territory and shed the blood of our fellow-citizens on our own soil."