Who will be our allies?
Our (UK) allied forces will be NATO countries of course. Wars tend to be triggered by events, eg an expansionist country like Russia, wanting to take territory it believes is rightfully theirs, breaking international law, push the envelope, or Hitler' early taking of Czechoslovakia in the lead up to WWII, (which was then followed by invading Poland), or Putin in 2014 annexing Crimea.
Initially countries don't respond, hoping it won't escalate, wanting to appease the aggressor in the vain hope of it not triggering adverse reaction. Democracies want to avoid war at all costs, because it leads to degradation of quality of life, economic ruin and death of a country's citizens. Hence The League of Nations, United Nations and now NATO, their stated aim is diplomacy and preservation of peace (no comment about trying diplomacy with Mr Putin!)
Who do you think we'll be at war with?
If Putin were to push it to the point of invading Poland, for example, being a NATO country, the next steps would be that NATO allies would come to the aid of Poland, as the pledge is that all allied countries are there to protect each other as a collective. Putin knows what would happen in that situation hence why he hasn't taken that action,
Will we be in the right?
It depends who you ask! a question like this always has to be seen in the context of what is happening, the sequence of events which lead to consequences - its never black-and-white.
UK Defence (note the word is Defence, the act of defending against an adversary, who commits an internationally illegal crime of invading a NATO country) is there to protect, being "in the right" is based on the motivation for our action. We wouldn't be waging war against Russia, we would be protecting Poland in that hypothetical situation. Then again if you were to ask Putin for his opinion, he would be in denial and justify his actions as being his ahem Special Military Operation, and as he doesn't recognise NATO, he blame it all on "The West". No point trying to reason or create logic with a madman.