KYIV, Ukraine — Every weekend for the past three years, the reservists of Ukraine’s 130th territorial defense battalion have gathered in forests and parks on the edge of Kyiv to train for the worst: a possible full-scale invasion from Russia.

At times, this seemed an unlikely prospect. That has suddenly changed with Moscow’s troops massed on the border and Ukrainians wondering what they will do — fight, flee, hide, adapt — if Russian forces roll across it.