1846: Bohemian crisis when Russia stations troops there. Germany, Sweden ally with New Rome against Russia.
The Anti-Russian War was fought during 1848-52 by the allied Germanies, Sweden, Canada, the Rum-Seljuks and New Rome. Although the population of the two Russias was higher than all its opponents combined, they soon faced themselves in trouble: After their standing armies were defeated by their opponents armed with the modern needle guns, it took a long time to arm, train and transport new soldiers to the front. Being cut off from imports, their soldiers were badly equipped. One century ago the Telegraph made Russia the most progressed nation on Earth, now the Chunkvophiles in the government who only thought about stability couldn't handle a war against industrialized nations. Unrests among the peasants and the occupied nations (Poland, Choresm) finally tipped the balance against Russia.
During the war, Socialist Britain sold arms to the allies, making a lot of money.
The war detailed
(It can be generally said, since the coalition powers didn't have a united command, that New Rome and the Seljuks did the fighting against South Russia and its Orthodox Balkan allies, while everyone else fought mainly against Novorossiya.)
1848, August: German-Swedish navies defeat Novorossiyan Baltic fleet.
1848, September: New Roman spies infiltrate the "Empty Circle", i.e. the area between Rockies and Sierra Nevada, Snake river and Colorado river, which Novorossiya claims. In practice, the area is home for a lot of communities - surviving Atlantean nations, sects, fled slaves and so on.
1848, October: Germans have conquered all of Bohemia and Moravia, except for the Russian garrison of Prague.
1848, November: In the cavalry battle of Nagykörös, New Romans defeat the Russians. Hungary west of the Danube is liberated.
1848, December: Seljuks march into Pontic lands occupied by South Russia.
Winter of 1848/49: Northern Finland liberated.
1849, January: Canadian skiing troops score their first major victory against Russian near OTL Fairweather Lake.
1849, April: Battle of Inowroclaw; with the help of uprising Poles, Germans throw Russian occupation army behind the Vistula.
In the battle of Medjugorje, the Serbs and Bosnians can stop the New Romans, using the difficult territory for their advantage. Emperor Benedetto decides to rather seek for the decision in Hungary, leaves Russia's satellites on the Balkan alone.
1849, May: Germans and Poles cross the Vistula at Sandomierz.
1849, July: Battle of Chelm. United armies of South Russia and Novorossiya beaten, have to retreat behind Bug river.
1849, August: Germans cross the Vistula, going into Prussia.
1849, September: Swedes take Tampere.
1849, October: German troops have reached Memel / Nyemen river.
1850, January: Battle of Drevesina (OTL Boise, Idaho).
1850, March: Battle of Békéscsaba. New Roman troops start reconquest of eastern Hungary.
1850, March: Germans start to besiege Riga.
1850, September: New Romans take Khust (Carpatho-Ukraine), concluding their conquest of Hungary.
1850: After the liberation of Prussia and Poland, the German politicians decide to strike at Russia's heart, driven by their wish for revenge. Emperor Benedetto whose troops just conquered Eger is surprised to hear they don't want to make peace.
1851, May: German-Polish-Czech troops take Minsk.
1851, August: Vanhakaupunki (OTL Helsinki) taken by Swedes.
1851, November: German main army destroyed (with help by general winter) at the little city of Moscow. The front has to be taken back behind Lake Peipus and the former east border of Prussia.
1852, February: Canadian commander Jacob Andrews (later president) manages to land on Vancouver island at night, the first step to conquer it.
Peace and aftermath
1852, May: In the peace of Constantinople, Prussia, Poland, and Greater Bohemia change into the German sphere of influence. Hungary becomes a satellite of New Rome. Seljuks take Pontus and Thrace back. The thinly settled hinterland of Russian Atlantis goes to Canada (which gains access to the Pacific) and German Atlantis, the Empty Circle (the lands between Snake river, Colorado river, Sierra Nevada and Rockies - about OTL Utah and Nevada) to New Rome. Finland and Estonia become Swedish again.
(Sorry if this sounds like anti-Russian wank. True, the coalition had better weapons like the needle gun, and fought an opponent, but still. In reality, the Russians will have won the one or other little battle not mentioned here - and the winners probably will prefer to stay silent too.)
1850s: After the humiliating defeat, Russia decides to modernize, building railroads and factories.