The Masaya Triple Layer tephra was deposited ~2100 years ago during a basaltic Plinian eruption of Masaya caldera, Nicaragua, and is one of few known examples of this extreme endmember of basaltic explosive volcanism. Masaya caldera is located approximately 25 km from Managua, the capital of Nicaragua, and a Plinian eruption presents a high potential risk to its population of 1 million people. Here we use geochemical and petrological tools to constrain the pre- and syn-eruptive physico-chemical magmatic conditions, in order to understand how low-viscosity basaltic magmas can erupt with Plinian style. By combining thermometric models with Rhyolite-MELTS simulations, we find that the Masaya Triple Layer magma was last stored between 1080-1100˚C and 21-42 MPa, with a pre-eruptive volatile content of ~2 wt.% H2O. A small phenocryst volume fraction of 0.1 crystallised under water-saturated conditions within a shallow magma reservoir located at 0.8-1.6 km depth. During ascent, rapid microlite crystallisation occurred and lateral velocity gradients across the conduit produced heterogeneous inter-mingling regions of varying crystallinity, between 20-50 vol.%. Crystal size distribution analysis shows substantial microlite crystallisation over a timescale of 1-5 minutes, which induced significant changes in viscosity and magma rheology during ascent, increasing the effective magma viscosity from ~10 Pa s to 106 Pa s and approaching the brittle fragmentation threshold. Comparable pre- and syn-eruptive conditions have been deduced for other examples of basaltic Plinian activity, indicating a common eruption mechanism. We find that the common requirements for a basaltic Plinian eruption are a low magmatic H2O concentration of ~2 wt.%, storage at moderate temperatures of 1080-1100˚C, and rapid crystallisation during magma ascent. Ultimately, these conditions contributed to the production of this highly hazardous Plinian eruption of Masaya.