same Union Army
No, it was not the “Union Army” .That army came into existence Feb. 28, 1861, and went out existence on May 26, 1865. It was the United States Army that that oversaw the “trail of Tears” and the Plains Indian wars.
"Cherokees! The President of the United States has sent me, with a powerful army, to cause you, in obedience to the Treaty of 1835, to join that part of your people who are already established in prosperity, on the other side of the Mississippi. . . . The full moon of May is already on the wane, and before another shall have passed away, every Cherokee man, woman and child . . . must be in motion to join their brethren in the far West." - General Winfield Scott, who oversaw the government eradication of American Indians from the south, later, a General in the Union Army
The same government, the same actors. Defend it however suits your needs. In the American Indians' case, the discovery of gold on their land being the reason for the federal government's human continental cattle drive.