To pass quickly downward
sink, topple, drop, stumble, trip, fall down, plunge, plummet, tumble, descend, go down, lower, totter, break down, cave in, decline, subside, collapse, buckle, crumple, settle, droop, slump, hang, regress, lapse, backslide, drop down, pitch, gravitate, come down suddenly, take a dive, take a nose dive, be precipitated, make a forced landing, fall flat, fall in, fold up, keel over, fall over, tip over, slope, slip, recede, relapse, abate, ebb, diminish, depreciate, decrease, flop*, take a spill*, take a header*.
Antonyms rise*, ascend, climb.
To be overthrown
submit, yield, surrender, succumb, be destroyed, be taken, pass into enemy hands, bend, defer, lie down, resign, capitulate, back down, topple, go down, go under, break up, get one's come-uppance*, fall to pieces*.
Antonyms prevail, endure*, resist.