Explorations of our galaxy, the Milky Way, are currently undergoing a revolution. Spearheaded by Gaia, large missions are measuring the motions, chemical compositions, and ages of stars more precisely and farther than ever before. Using these data, for the first time we have been able to unambiguously isolate stars born in smaller galaxies that have been assimilated by the Milky Way throughout its history. Yet open questions abound following this milestone achievement: "How are the different progenitors of the Milky Way related?" "How has their arrival impacted the stars and dark matter already in the Milky Way?" I will discuss how the answers to these questions can be gleaned from precise orbital histories of stars in the Milky Way halo, and what they imply for the intertwined quests to understand the physics of galaxy formation and the nature of dark matter.