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Recent studies using Gaia DR2 have identified a massive merger in the historyof the Milky Way (MW) whose debris is markedly radial and counterrotating. Thisevent, known as the Gaia-Enceladus/Gaia-Sausage (GE/GS), is also hypothesizedto have built the majority of the inner stellar halo. We use the cosmologicalhydrodynamic simulation Illustris to place this merger in the context of galaxyassembly within $\Lambda$CDM. From $\sim$150 MW analogs, $\sim 80 \%$ haveexperienced at least one merger of similar mass and infall time as GE/GS.Within this sample, 37 have debris as radial as that of the GE/GS, which we dubthe Ancient Radial Mergers (ARMs). Counterrotation is not rare among ARMs, with$43 \%$ having $>40 \%$ of their debris in counterrotating orbits. However, thecompactness inferred for the GE/GS debris, given its large $\beta$ and itssubstantial contribution to the stellar halo, is more difficult to reproduce.The median radius of ARM debris is r$_{*,deb}\simeq 45$kpc, while GE/GS isthought to be mostly contained within $r\sim 30$ kpc. For most MW analogs, afew mergers are required to build the inner stellar halo, and ARM debris onlyaccounts for $\sim 12 \%$ of inner accreted stars. Encouragingly, we find oneARM that is both compact and dominates the inner halo of its central, making itour best GE/GS analog. Interestingly, this merger deposits a significant numberof stars (M$_*\simeq1.5 \times 10^9 M_\odot$) in the outer halo, suggestingthat an undiscovered section of GE/GS may await detection.

Cosmological insights into the assembly of the radial and compact stellar halo of the Milky Way