You dont have to look any further than a Google of 'Nato countries' , then images, then look at the BBC image in purple and orange that shows NATO expansion since 1997.
Look at the physical position of Belarus and Ukraine. He's already lost alot of ground to NATO and his red line is Belarus and Ukraine.
Ukraine was headed towards NATO and that's a step too far for him. End of complicated theories on why this is happening.
Not saying he's right, he's not, any country have have any alliance they wish. But from his perspective NATO have gone too far and upset the balance.