Not many countries volunteered their soveriegnty to the British Empire, nor the Russian or German Empires, or even the Austro-Hungarian ones, or the Ottoman, or the Mongol. All of these empires were created through force of arms and involved denial of self-determination of the conquered peoples.

As far as I aware, there is no state wide secessionist movement, although two towns did consider seceding from the state (not the nation) over a state wide property tax.

Although there was meddling in Mexico’s affairs, the US was merely enforcing the wishes of the American and European settlers in Mexico’s north west colonies who did not want to be part of a corrupt Mexican regime but wanted self-determination within the Union. That is not an imperial action.

As far as Latin America is concerned, the Monroe Doctrine was a pretty comprehensive statement of opposition to colonialism in the Americas. In Panama, the US has a mixed history, one of militarily suppressing independence at the invitation of the Columbian government, then supporting Panamanian independence when it looked like local administration were going to agree to their treaty demands for the proposed Panama canal. Truman didn’t connive anyone, Panama gained independence while Roosevelt was in power in 1903. Panama were not entirely a client state, but neither were they completely independent. The US never claimed soveriegnty over Panama (just the canal zone), only extracted a one-sided treaty. Not nice, but not entirely imperial. This is probably the most imperial action that the US ever conducted. Still doesn’t make them an empire though, especially not today since they gave up the canal and their military bases in 1999.

The worst thing the US government did in my opinion was the invasion of the Confederacy in 1861. That might be considered an imperial action, since the Southerners clearly didn’t want to be part of the Union any longer. However under international conventions, that was a civil war, as the Confederacy gained no formal independence.

Soveriegnty over Utah was exchanged from Mexico to the US at the end of the US-Mexican War in 1848, Utah was never formally independent. It can’t have been too bad, because they actually tried to join in 1849 as the State of Deseret covering a land area much larger than present day Utah, but the US government were wary of admiting such a large state and were opposed to the policy of polygamy. The Church of LDS formally renounced polygamy as a condition of admitance, but they still did it voluntarily. They did not declare their independance from Mexico or from the US. The military response to the LDS authorities was entirely a domestic response. Not pretty perhaps, but not imperial in nature.