  After a decade of great progress in understanding gas flow into, out of, and
through the Milky Way, we are poised to merge observations with simulations to
build a comprehensive picture of the multi-scale magnetized interstellar medium
(ISM). These insights will also be crucial to four bold initiatives in the
2020s: detecting nanohertz gravitational waves with pulsar timing arrays
(PTAs), decoding fast radio bursts (FRBs), cosmic B-mode detection, and imaging
the Milky Way's black hole with the Event Horizon Telescope (EHT).
