This is the second time President Obama has made 'red line' statements. He needs to BUILD his relationship with Putin, not constantly break it down imho. I think it's mad to cancel the G8 as that's a great opportunity to put pressure on Putin to withdraw troops on the Crimea border by the other member nations. It's just typical of the US and the UK to make war type threats rather than use verbal diplomacy. Of course Putin is a tricky character, all the more reason to try and reason with him and give him some ground to back track without losing too much face, rather than issue deadlines and military type threats. I note Hague is taking the more diplomatic route today... according to the BBC Live Feed:

http://www.bbc.co.uk/news/world-europe-26413953

I think this is much more to do with money in the longer term. Russia pays Ukraine for the use of the Crimea port of Simferopol for its northern ship fleet. Russia is also owed over a £billion (or equivalent) by the Ukraine for use of the gas pipelines and has defaulted on some payments. 

The biggest threat to Europe and us, is the power Russia would have over us by controlling the gas pipelines, which all currently run through the Ukraine. 

Ukraine has reminded me of the Hungarian uprising in 1956 when I was at school and we took in a Hungarian refugee called Marta. She left after a while and I wondered what happened to her.