Military invasion refers to a military operation in which armed forces of one geopolitical entity enter the territory of another such entity, often with the intent to occupy the land. It may occur within the context of a war or it may be an independent conflict. Invasions often profoundly change the culture of the invaded region and sometimes the political landscape. They are normally accompanied by violent conflict and often result in the transformation of the defeated faction into a client state, often with requirements to pay reparations or tribute to the victorious regime.
Various reasons have been advanced for the mounting of an invasion, including restoration or re-establishing control over territory lost in a previous conflict; religious idealism; politics of national interest; the pursuit of enemies; policies of empire building; the acquisition of natural resources and transportation routes; quelling destabilizing or unconscionable domestic conflict within or between neighbors; and retaliation for real or perceived actions. Invasions are usually conducted on a large scale and are characterized by military assaults, largely involving infantry troops, though air and sea strikes or cruise missiles launched from ships at sea are sometimes used to “soften up” the target, and in some cases other means eliminate the need for a ground assault altogether, as in the 1945 atomic bombing of Hiroshima and Nagasaki, which made it unnecessary for the Allies to invade the Japanese home islands with infantry troops.
Depending on the situation, invaders may try to mitigate the impact of an invasion by creating a series of spaced fortifications along the border between themselves and the defending nation, in order to delay the invasion until the defender has had time to organize a defense, or they can also be used to interdict the invasion’s supply lines, as was the case with the Maginot Line for the Allies on D-Day in June of 1944.