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The United States didn't immediately annex Texas because Northerners opposed to slavery objected to the annexation of more slave territory and didn't want slave states to outnumber free states. Many Americans also feared that annexation would lead to war with Mexico. ... Mexico refused to negotiate on the US offer.
Following Texas' successful war of independence against Mexico in 1836, President Martin van Buren refrained from annexing Texas after the Mexicans threatened war. ... His efforts culminated on April 12 in a Treaty of Annexation, an event that caused Mexico to sever diplomatic relations with United States.
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