Frum combines two talking points:
There is only one last non-military stop on this train: President Obama’s initiative to organize so-called “crippling” sanctions against Iran.
These sanctions would penalize the firms that sell, carry and finance the half-million tons of gasoline that Iran must import every month. (Incredibly, this huge oil exporter and aspiring nuclear power refines only about half the gas it needs.) Such firms are vulnerable to international pressure: Two of the three Swiss firms that provide the bulk of Iran’s gasoline have substantial investments in Canada, for example. If Canada joins the sanctions regime, Canada can bring great pressure to bear on these suppliers and thus upon Iran.
To sustain sanctions over any length of time, however, will require international co-operation, especially from Russia, China and India. Will that co-operation be forthcoming? So far, the record is not promising. But if those countries understand that the final destination of the Iranian effort is an Israeli military strike on Iran, maybe they will rethink. For that reason, the whole world has an interest in enhancing the credibility of Israeli action. For that reason, the campaign to penalize and demonize Israel for its actions in Lebanon and Gaza is an affront to world peace. Only an effective Israel can believably threaten the strike that will incentivize Iran’s trading partners to join the U.S. economic campaign.
So we have gone from denying outright any moral problems associated with the Gaza attack to arguing that, regardless of what actually happened, the whole world should back Israel because of the danger of Iran and the fact that Israel's threatened military strike is the only real lever we have.
Well: let's see how China might view this. They might ask: why should we care if Iran goes nuclear? Or rather: do we care enough that we are prepared to initiate a global terror war with utterly unforeseen consequences rather than tolerate a nuclear - or a latent nuclear - Iran? I think the Chinese would understandably say: nope. Ditto the Russians. They're happy to watch America squirm because of Israel's insistence on either crippling sanctions or war (with no concessions, of course, with respect to the Palestinians).