【Invasion】 , 【incursion】 ,  【raid】 ,  【inroad】  are comparable when meaning an entrance effected by force or strategy.

【Invasion】 basically implies entrance upon another′s territory with such hostile intentions as conquest, plunder, or use as a basis of operations. In nonmilitary use it may imply encroachment, trespass, or an intrusion that involves an aggressive or hostile purpose.

Sometimes it implies no more than entrance with or as if with a rush by a horde or crowd.

【Incursion】 , especially in military use, carries a stronger connotation of suddenness, unexpectedness, or haste than  【invasion】 ; it often also suggests an immediate end and a quick withdrawal when the end is achieved. In its extended sense  【incursion】 applies chiefly to an 【invasion】 in large numbers of something dreaded or harmful, undesirable, but not necessarily inimical.

【Raid】 , frequent in military use for a swift, sudden 【invasion】 (as of cavalry or of air forces) may or may not suggest more preparation, more strategy, and more fury in attack than  【incursion】 , its close synonym.

In international law, however,  【raid】  is applicable specifically to an 【incursion】 of armed forces that are unauthorized or unrecognized by any state into a country that is at peace; thus, an  【incursion】  of armed persons on one side of a border or boundary line into the adjoining country for a predatory or hostile purpose is technically a  【raid】 . In its extended use  【raid】  applies to a sudden descent or a flurry of activity intended usually to obtain the use, control, or possession of something; thus, officers of the law conduct a  【raid】 upon a gambling resort or a place where liquor is illicitly made or sold to obtain evidence and arrest offenders.

【Inroad】 may apply to a sudden hostile 【incursion】 or a forcible entering, but the term is also applied to an 【invasion】 that involves encroachment or advance especially at the expense of someone or something.