The Early Merger that Made the Galaxy's Stellar Halo
  The last two years have seen widespread acceptance of the idea that the Milky
Way halo was largely created in an early (8-10 Gyr ago) and massive ($> 10^{10}
M_\odot$) merger. The roots of this idea pre-date the Gaia mission, but the
exquisite proper motions available from Gaia have made the hypothesis
irresistible. We trace the history of this idea, reviewing the series of papers
that led to our current understanding.
