The sanctions weren't intended to prevent military conflict- the threat of sanctions was. The threat failed.

Now the question should be, will following through on the threat mitigate the harm now or in the future? Against Russia, my guess is no. They also haven't been successful against Iran or any number of other counties.

It might be said that the effectiveness of sanctions is entirely reliant upon being able to keep a weak nation weak, rather than preventing a strong nation from preying upon weaker neighbors.