During the French Revolutionary War, the Rhine frontier was achieved, only to be lost twenty years later in 1815 (after the defeat of Napoleon). That loss, however, proved (albeit reluctantly) acceptable because the Rhineland was merely France’s hinterland. The heartland was France’s remaining (pre-war) territory, known as the “Hexagon” from its symmetric shape. In 1871 (after her defeat in the Franco-Prussian War) France was deprived of the north-east corner of the Hexagon, namely Alsace-Lorraine. That was totally unacceptable because it was an integral part of the French Hexagon, whose symmetry was destroyed by its loss. On French maps, Alsace-Lorraine was coloured black, as occupied France. In 1914, France fought and won a war of revenge resulting in the reconquest of the territory in 1918. By contrast, France’s loss of Algeria in 1962 was acceptable because although Algeria had been made part of Metropolitan France in 1842, it was merely a hinterland of the sacred Hexagon heartland.
After defeat in the Second World War, Germany lost its eastern territory, and the remainder was partitioned. Eventually, West Germany accepted the loss of territory east of the Oder-Neisse line, but not the loss of East Germany, because the latter encompassed the former capital (Berlin). Therefore, West Germany campaigned for reunification with the Oder-Neisse Line as eastern frontier. This was because West and East Germany constituted the heartland, whereas the lost eastern territory (long disputed with Poland) was merely the hinterland. Reunification was eventually achieved in 1991 after over forty years of struggle and the Oder-Neisse line became the recognised eastern frontier of united Germany.
The country was partitioned (against the wishes of the great majority of its inhabitants) in 1922. This was effected by the United Kingdom government to retain the Loyalist Protestant population in North-East Ireland (the Six Counties). Irish Nationalists have never and will never accept this partition, because it deprives their country of part of its inalienable heartland - the whole Emerald Isle.
The country achieved unification in 1861. It lacked however, territories regarded as part of its heartland which encompassed the whole of the Italian peninsula south of the Alps. The claimed lands were known as “Italia Irredenta” (Unredeemed Italy). The country fought and eventually regained these areas. Venetia was annexed in 1866; Rome in 1870; South Tyrol and Trieste in 1919.
The heartland is England whereas Ireland, Scotland and Wales are the hinterland. That is why most of Ireland was allowed to secede in 1922 and Scotland given the opportunity to do likewise in 2014 (by referendum).
The heartland is the contiguous 48 state territory stretching from “Sea to Shining Sea” (Atlantic to Pacific Ocean). That is why the Zimmerman Telegram of 1917 helped provoke the United States into declaring war against Germany. This promised German assistance in annexing Arizon, New Mexico and Texas in exchange for a Mexican declaration of war against the USA. Even though it was rejected, that offer provoked implacable hostility, because it involved the threatened loss of part of the American heartland.