It may very well be that sanctions will not work, but they have to be tried, as do any and all diplomatic means.
Would you really prefer the goverments of the UK, USA and the various countries of the EU plus NATO to have started World War III instead?
It remains to be seen whether Putin will realise that the sanctions against Russia will bring the country to its knees, or whether like Hitler he will not be deterred. But no-one tried sanctions against Hitler, did they?
I do remember that sanctions did not stop Apartheid in S. Africa either, but we may be luckier this time round.
And forgive me from pointing out, that by the time Hitler marched into Poland it was already too late. He should have been stopped when he marched into Austria.