Short of a full scale invasion of Europe I find it hard to imagine a Russian act that would cause European leaders to start a war. So the question of what would

Short of a full scale invasion of Europe I find it hard to imagine a Russian act that would cause European leaders to start a war. So the question of what would count as casus beli is moot. Russia has over 1M active military personnel, with millions more in reserve as well as an enormous arsenal of weapons ranging from old to absolutely cutting edge (not to mention nuclear ones). They are easily the biggest, most well funded military force in Europe by a significant margin.