O'Sullivan's first use of 'manifest destiny' supported annexing which republic?

O'Sullivan's first use of “manifest destiny” supported annexing the Republic of Texas.

John L. O'Sullivan used the phrase in his 1845 essay “Annexation,” published in the Democratic Review. He argued that bringing Texas into the United States was justified by a providential mission for the country to expand across the continent. Texas had declared independence from Mexico in 1836 and operated as an independent republic before joining the Union.

Congress approved Texas's annexation in 1845, and it became the 28th U.S. state in December of that year. The issue was politically explosive because Texas permitted slavery, raising fears that annexation would intensify sectional conflict.

The phrase is often associated with O'Sullivan's later argument over Oregon. In a December 1845 article, he invoked “manifest destiny” to defend the U.S. claim to the whole of Oregon Country. That was his second use, not the first; the first concerned Texas.