First of all, I would like to point out one historically significant development in the modern Russo-Ukrainian War: there are reports that the Ukrainian Army routed the 4th Guards Tanks — the legendary Kantemyr division.
Now let’s get back to the historical development of Malorossicism — the longstanding idea that Ukrainians are not their own independent nation but rather “Little Russians” — an inferior branch of the “Russian World” incapable of self-government. The pendulum-style swings between support for Ukraine as “brothers” and their domination as “Little Russians” can be seen clearly in the latter half of the 17th and first decades of the 18th centuries.
The first main development is a high-water mark of Russo-Ukrainian relations, the support of Tsar Mikhail I for the Ukrainian uprising under Bohdan Khmelnitsky. Khmelnitsky and the Zaporozhian Cossacks first rose up against Polish rule in 1647 and enjoyed initial successes. However, the latter part of the revolt was marked with military reverses, the withdrawal of a key ally (the Crimean Tatars, who up to that point provided the cavalry support that helps account for why this uprising held while the ones before had failed), and some of the most notorious and widespread pogroms in European history.
In search for new allies, Khmelnitsky negotiated with several parties but eventually landed on Tsar Mikhail. The Treaty of Pereiaslav established an autonomous Cossack state — the Hetmanate — under Russian protection. This secured the success of the uprising and provided a base for Russian Great Power status. As historians have put it, Russia=Moscow+Ukraine.
The initial feelings of solidarity, however, deteriorated over the next half-century. After Khmelnitsky’s death, successive Tsars worked to increase Russian control over the Hetmanate and diminish its autonomy. The pendulum fully swung in the other direction by 1708. Russia under Peter I had been fighting Sweden under Charles XII in the Great Northern War for almost a decade by that point. After an initial massive victory at Narva, Charles had been largely bogged down with attempts to retake newly founded Saint-Petersburg and also to pacify Poland, where he kept willing battles but was unable to enforce his rule. But by 1708 he was ready to invade Russia again.
Tied up by the Swedish invasion moving towards Moscow, Peter was unable to assist Hetman Ivan Mazepa resist the allied Polish invasion to the south, which was the last straw — the main obligation of Russia was specifically to defend Ukraine against Poland. Mazepa then threw in with Charles, who, finding the road to Moscow effectively impossible to use given a scorched earth policy and refusing to accept retreat as the best option, chose to push southward to join forces with Mazepa.
Peter responded using largely the same strategy that Putin is using now in Ukraine — decimating the cities in order to pacify the population. After defeating Cossack forces at Baturin, the city was destroyed and the population massacred. Similar punitive attacks elsewhere dried up support for the Cossacks and the Swedes, and Peter prevailed over Charles at the battle of Poltava in 1709, effectively deciding the war, although Charles and the Swedes would continue fighting until 1720.
Peter then moved to ensure that the Hetmanate would never have the power to do so again. It was here that Malorossicism became formal Tsarist policy, with Ukrainian language subordinated to Russian, notably in the dissolution of the Hetmanate and the establishment of the Little Russian Collegium, and in forbidding the publication of texts in Ukrainian from the 1720s.