A good PoD can be that Britain declare war on Russia for Poland. Russia and Britain were on edge of war in 1863 because of January Uprising.
IIRC there was some sort of insulting message or ultimatum sent from Palmerson to Alexander II that wasn't delivered because British ambassador to Russia Napier pretended to be ill ( later it was cancelled by Palmerson himslf). Had Napier been ill indeed message would be delivered almost certantly ( only Napier himself had enough authority to delay PM messages). Russia would certantly deflect this message and Britain would declare war on Russia( even if my memory fails me such an ultimatum certaintly COULD be sent from Palmerson to Alexander II).
Britain and USA we not the closest friends in that period and if say Britain attacs Lesovsky Fleet in US waters( that was sent in OTL to USA in case of war with Britain) than USA will probably join Russia in its war with Britain. The war of Britain and USA can be caused by multiple other reasons ( say Britain suppor of CSA is more abrupt ). In any case if Russia and USA have wars with Britain they will surely conclude formal alliance.
This conflict can be
A) war between USA and Russia on one side and Britain and CSA( separately or in an alliance).
B) First World War if Britain allies in Europe also attack Russia. If Austria and France attack Russia, Prussia and Italy will probably side with Russia and USA( Prussia has alliance with Russia and plans to fight with Austria and France, Itally joins whoever fights with Austria). Ottamans can also join Briatian. In this case we have WWI with following sides: Russia, USA, Prussia and Itally v. s. Britain, CSA, France and Austria( and probably Ottomans).
IIRC there was some sort of insulting message or ultimatum sent from Palmerson to Alexander II that wasn't delivered because British ambassador to Russia Napier pretended to be ill ( later it was cancelled by Palmerson himslf). Had Napier been ill indeed message would be delivered almost certantly ( only Napier himself had enough authority to delay PM messages). Russia would certantly deflect this message and Britain would declare war on Russia( even if my memory fails me such an ultimatum certaintly COULD be sent from Palmerson to Alexander II).
Britain and USA we not the closest friends in that period and if say Britain attacs Lesovsky Fleet in US waters( that was sent in OTL to USA in case of war with Britain) than USA will probably join Russia in its war with Britain. The war of Britain and USA can be caused by multiple other reasons ( say Britain suppor of CSA is more abrupt ). In any case if Russia and USA have wars with Britain they will surely conclude formal alliance.
This conflict can be
A) war between USA and Russia on one side and Britain and CSA( separately or in an alliance).
B) First World War if Britain allies in Europe also attack Russia. If Austria and France attack Russia, Prussia and Italy will probably side with Russia and USA( Prussia has alliance with Russia and plans to fight with Austria and France, Itally joins whoever fights with Austria). Ottamans can also join Briatian. In this case we have WWI with following sides: Russia, USA, Prussia and Itally v. s. Britain, CSA, France and Austria( and probably Ottomans).