An antagonist is an enemy or the story’s villain.  The antagonist of The Tamarack Tree is the Union Army, and specifically Union soldiers.  This is because the novel takes place in Vicksburg, Mississippi during the Civil War.  Vicksburg is in the South, but it has to be taken by the North in order to divide the South.  Rosemary, the main character, has to hide inside a neighbor’s house to avoid the shelling, or bombing.  When she is rescued by a group of Southern soldiers, she is relieved that the siege is over and she can come out.