Forthweg was a kingdom in northern Derlavai. It was bordered on its eastern side by Algarve and on its western by Unkerlant. Forthweg has historically often been ruled by one or both of its larger neighbors in its history, though in the days of Plegmund, Forthweg was strong enough to make inroads into both of those kingdoms at the same time. It was also once ruled by the Kaunian Empire, and Forthweg had a large Kaunian population until the Derlavaian War. These Kaunians were on very tense terms with their indigenous Forthwegian neighbors.

After the Six Years' War, the Kingdom of Forthweg was reconstituted when its eastern half was liberated to punish Algarve. Its western half soon seceded from Unkerlant, taking advantage of the chaos gripping that kingdom in the aftermath of the Twinkings War.

When Algarve annexed the Duchy of Bari following the death of Alardo, Duke of Bari Forthweg, along with Valmiera, Jelgava, and Sibiu, immediately declared war against Algarve. Its army fared badly against Algarvian forces and was driven deep inside their territory. When Unkerlant suddenly invaded it from the West, Forthweg was once again completely overrun between its two traditional enemies.

Soon after, Algarve began its invasion of Unkerlant by overrunning Unkerlant's Forthwegian holdings. Forthweg would play a pivotal role throughout the remainder of the war. The Algarvians, hoping to take advantage of Forthwegian animosity towards Unkerlant, raised Plegmund's Brigade. Algarvians also gathered Forthwegian Kaunians for use in their so-called "special magecraft". Many Forthwegians assisted in this genocide.

When Unkerlant drove Algarvian forces back into Forthweg, Forthwegians in Eoforwic revolted against their Algarvian overlords, hoping to establish an independent kingdom before their other great enemy again overran them. At this they failed, and under King Beornwulf I, Forthweg was dominated by Unkerlant, though it was not annexed outright.