Mannerheim's Finland takes inspiration from both the nationalistic idea; Greater Finland (Suur-Suomi) and Mannerheim's ambitions and desire to intervene in the Russian Civil War during and after the Finnish Civil War on the side of the White Movement.
In our timeline, Carl Gustaf Emil Mannerheim, commander-in-chief of the White Finnish Army during the Finnish Civil War was disappointed with the denial of his plans to invade Russia and support the White Movement there, stood down and vacated in Sweden. He returned in December of 1918 upon request as Regent of Finland and represented Finland during the peacetalks, successfully obtaining recognition of Finland's independence from the Western Allies.
In this timeline, during his vacation, Mannerheim received confirmation of support from British Ambassador for Sweden; Sir Esmé Howard. Howard told Mannerheim that the British are already supporting the White Movement in Northern Russia and are more than happy to support Mannerheim's efforts to push Finland further into Russia as anti-Bolshevik forces. Britain decided that Finland would become it's bastion of operations and would support Monarchist movement there in accordance to Mannerheim's belief that Finland needed a strong leader external to partisan pressure. With this, Britain supported Mannerheim to become the King of Finland.
Following Mannerheim's return to Finland, he gained support from the senate to aid White Russia and invaded Karelia in preparation to take the strategically important Petrograd. The campaign was a success and Finland, with Allied forces, occupied the Kola Peninsula and the three Isthmuses that stretched between the head of the Gulf of Finland and to the White Sea. The forces awaited for reinforcements from the Estonian volunteers that were mobilizing from the south led by ex-imperial General Nikolai Yudenich. The forces successfully assaulted the city and faced little organised opposition by Red Army forces. After 5 days of fighting, the city fell and the occupation of the city was defended for the Summer from Red Army reinforcements until the Treaty of Petroskoi. There, Lenin ceded the lands to Finland and recognised Estonia's independence ending the conflict. It was indeed a monumental blow to the Bolshevik movement leading to the armistice between the Russian Whites and Reds, leaving a border conflict between the Ural Mountains. Mannerheim became King of Finland, Kolchak led the newly formed Republic of Siberia and Lenin the Russian Soviet Federative Socialist Republic.