Subduction is a key process for the plate tectonic cycle and is responsible for the bimodal composition of the Earth’ crust. Whereas active subduction zones can be directly observed at many places, their initiation and the early evolution of the associated volcanic arc can only be studied from the geological record. One key location to study the geological processes related to subduction initiation and subsequent arc emergence and maturation is the Izu-Bonin-Mariana (IBM) subduction zone system in the Western Pacific. Here, a unique record of rocks that formed during the earliest stages of the newly formed subduction zone are preserved in the forearc. However, much less was known about the spatial extent of these lithologies and thus the temporal evolution and the dynamics of subduction zone initiation in the IBM system. In 2014, IODP Expedition 351 added an important perspective by drilling in a rear-arc location, thus complementing the geological record across the proto-IBM arc. In this talk, I will provide a synthesis of the scientific achievements gained through this expedition. The technically challenging drilling recovered a 1.45 km-long section of hemipelagic and volcaniclastic sediments, and 150 m of oceanic igneous crust. New oceanic crust formed analogous to the so-called forearc basalts during subduction initiation, and age and composition of the basaltic crust allow us to constrain the dynamics of subduction zone initiation. The volcaniclastic sediments above provide us with important insights into the compositional and temporal evolution of the volcanic arc over its full lifespan.