From The Collaborative International Dictionary of English v.0.44 [gcide]:
Usage: Invasion is the generic term, denoting a forcible entrance into a foreign country. Incursion signifies a hasty and sudden invasion. Irruption denotes particularly violent invasion. Inroad is entry by some unusual way involving trespass and injury.
From WordNet (r) 2.0 [wn]:
From Moby Thesaurus II by Grady Ward, 1.0 [moby-thes]:
67 Moby Thesaurus words for "invasion": adoption, aggression, air attack, air raid, air strike, appropriation, arrogation, assault, assumption, attack, blitzkrieg, boarding, breach, drive, encroachment, entrance, entrenchment, escalade, fire raid, foray, impingement, imposition, incursion, infestation, infiltration, influx, infraction, infringement, injection, inroad, insinuation, interference, interjection, interloping, interposition, interposure, interruption, intervention, intrusion, irruption, lousiness, obtrusion, offense, offensive, onslaught, overrunning, overspreading, overswarming, plague, playing God, raid, ravage, razzia, saturation raid, scaling, seizure, shuttle raid, storming, swarm, swarming, teeming, transgression, trespass, trespassing, unlawful entry, usurpation, violation
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