Since the threat of sanctions clearly failed to deter Russia’s Crimean intervention, we are now in a situation in which something has gone “awry”. Here, sanctions are even less likely to be effective.
Considering the present targeted travel bans and asset freezes only, the “transmission mechanism” requires that they harm Russia’s political elite enough to impose costs on Putin; no one regards this as likely.
Russia’s weak economy may make more effective. But to inflict damage directly would require more extensive sanctions; their indirect effect of further eroding business confidence in Russia may, however, still be costly.
That Putin’s actions seem to enjoy widespread Russian support suggests that he is not politically vulnerable. Indeed, sanctions may strengthen him by generating a “rally round the flag” effect which unites Russia against foreign foes. (By contrast, in apartheid South Africa, the African National Congress’ successful call for sanctions reinforced its position.)
If the EU and US wish to resist Russian actions in Ukraine, then their response must almost certainly include sanctions. On their own, however, they do not seem to threaten significant harm to Russian decision makers. Beyond this, there is a question of what the US and EU goals are.
My guess is that Crimea is gone: I cannot see the US and EU willing to bear real economic costs to oppose the right of Crimeans to determine their political future, now a fait accompli. If, however, Russian activity continues to destabilise eastern Ukraine, Putin may discover that the slow US and EU response which he likely sees as weak may be bolstered by a willingness to impose the so-called “stage three” round of more punitive economic sanctions.