The term Norwegian Realm (Old Norse: *Noregsveldi, Bokmål: Norgesveldet, Nynorsk: Noregsveldi or Noregsveldet) and Old Kingdom of Norway refer to the Kingdom of Norway's peak of power in the 13th century after a long period of civil war before 1240. The kingdom was a loosely unified state including the territory of modern-day Norway, modern-day Swedish territory of Jämtland, Herjedalen, Ranrike (Bohuslän) and Idre and Särna, as well as Norway's overseas possessions which had been settled by Norwegian seafarers for centuries before being annexed or incorporated into the kingdom as 'tax territories'. To the North, Norway also bordered extensive tax territories on the mainland. Norway, whose expansionism starts from the very foundation of the Kingdom in 872, reached the peak of its power in the years between 1240 and 1319.

A unified realm was initiated by warrior bands that had coalesced around Harald "Shockhead" Hálfdanarson—later known as King Harald Fairhair—in the 9th century. Before Harald, the coastal stretch from Vík (the Oslofjord area and the Skagerrak coast) in the south to Hálogaland (the northern Atlantic coast, roughly from Namdalen to present-day Troms, and beyond) consisted of shifting alliances and persistent hostility among groups whose political loyalties were tied to smaller fylki (districts). As far as historians can determine, none before him had ever asserted that these separate districts ought to form a single political unit—the entity later called the Kingdom of Norway. That claim built, however, on an already older conception of Norðvegr (the North Way) as a maritime transit route and a shared sociocultural landscape, inhabited by people known as "Northmen"—the common appellation for "Norwegians" in each of the North Germanic languages (to wit, Norwegian, Danish, Swedish, Icelandic, and Faroese)—alongside other peoples, notably the Sámi.

At the peak of Norwegian expansion before the civil war (1130–1240), Sigurd I led the Norwegian Crusade (1107–1110). The crusaders won battles in Lisbon and the Balearic Islands. In the Siege of Sidon they fought alongside Baldwin I and Ordelafo Faliero, and the siege resulted in an expansion of the Kingdom of Jerusalem. Leif Erikson, an Icelander of Norwegian origin and official hirdman of King Olaf I of Norway, explored America 500 years before Columbus. Adam of Bremen wrote about the new lands in Gesta Hammaburgensis ecclesiae pontificum (1076) when meeting Sweyn I of Denmark, but no other sources indicate that this knowledge went farther into Europe than Bremen, Germany. The Kingdom of Norway was the second European country after England to enforce a unified code of law to be applied for the whole country, called Magnus Lagabøtes landslov (1274).

The secular power was at its strongest at the end of King Haakon Haakonsson's reign in 1263. An important element of the period was the ecclesiastical supremacy of the archdiocese of Nidaros from 1152. There are no reliable sources for when Jämtland was placed under the archbishop of Uppsala. Uppsala was established later, and was the third metropolitan diocese in Scandinavia after Lund and Nidaros. The church participated in a political process both before and during the Kalmar Union that aimed at^{[clarification needed]} Swedish side, to establish a position for Sweden in Jämtland. This area had been a borderland in relation to the Swedish kingdom, and probably in some sort of alliance with Trøndelag, just as with Hålogaland.

From the 600s Western Norwegian fish farmers began an exodus to the nearby islands in the North Sea, Orkney and Shetland, and then later to the Western Isles, like the Hebrides and the Isle of Man, and westward to the Faroe Islands, Iceland, and Greenland. Some of these islands were inhabited when the Norwegians arrived, but the local population was displaced or assimilated by the Norwegian immigrants.

Consistently, the islands' populations had a Norwegian ancestry, who kept in touch with the homeland over the North Sea. These Norwegians had their own chiefs or kings in the Norwegian tradition, subject to Norwegian royal power when it eventually developed a centralized state. Often, Norwegian kings had enough to contend with on the mainland, so the local power in the villages was often in the hands of local jarls who operated on behalf of the king.

Holdings in Sweden were in varying degrees Norwegian. By the 9th and 10th centuries, it is reasonable to assume that the population of Båhuslen, Jämtland and Herjedalen had no national affiliation to Norway, Svealand, or Götaland. It lay to the increasingly centralized monarchy to create this, which had to consolidate its right in the border areas above the neighboring kingdoms. Norway was then the first to integrate these areas into its kingdom.

The civil war era began in 1130 and ended in 1240. In this period of Norwegian history, some two dozen rival kings and pretenders waged wars to claim the throne. The Civil War period can be divided into phases: the first phase was sporadic strife between the kings from 1130 to the second phase, where there were extensive battles between them from 1160 to 1184; and the final phase in which the Birkebeiners defeated the rest in 1240.