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Your information is incomplete. China and Russia are doing a tad bit more than "diplomatic support".
Chinese and Russians have quietly, without committing any boots on the ground, made any sort of military campaign against Iran pretty much unviable. This is their supply lines, trade network, Belt and Road Initiative hanging in the balance so they're doing what's necessary, but no more.
The January insurrection by Baluchi, Kurdish and MEK groups (trained, armed and directed by Mossad and CIA), was subdued in large part thanks to Russian and Chinese tech to disrupt Starlink communcations and their ability to coordinate their violent activities. Turkish intelligence also assisted Iran in identifying and giving the movements of these groups.
After January, and the 12-Day war of June 2025, it has become clear to Iran (and to Russia and China) that the United States of Israel are now fully intent on bringing down the Iranian regime. They now want to see through that Iran goes the way of Iraq, Libya etc and is balkanized.
Incidentally, whatever reformist movement existed in Iran prior to these developments, and there has indeed been reformist presidents like Khatami who were seeking to normalize relations with the West, that's all dead and buried at this point. Nobody has any illusions about anything else than their adversaries wanting to reduce the country to rubble, and so there's been a rallying to support the current regime.
The 12 day war Iran used a lot of home grown anti air defense systems. Since then they've accepted and brought in Russian integrated air defense systems and Chinese BeiDou real-time satellite targeting, accurate to 2 meters, long range radar, intelligence, surveillance and guidance systems, air defense systems, and materials and components for missile production. Russia and China have ramped up their support significantly.
This is game-changing. You have to assume that the carrier groups are now viable targets. Add to that, they may even have reliable radar profiles of the B2 Spirit. Also, they can detect launches at 700 km range. For missiles going 1000 km/h that gives them about half an hour to react.
This has been reported on by Alastair Crooke, as well as Max Blumenthal of The Grayzone.
Offensively, of course they still have the capability to overwhelm missile defense systems in Israel and American bases, and then strike at specific targets with high accuracy, and they were hitting Israel hard enough for them to sue for a cease-fire.
Israeli news media (which has always been more honest and thus more informative than Western news media) has reported on what Bibi came to convey to Trump in their latest meeting. Essentially Israel has moved on from the nuclear issue. They are now demanding that the US takes out Iranian missile capabilities, and if there's to be any kind of negotiated settlement, the Iranians have to give up their missile arsenal.
Their missile capability is completely decentralized. Meaning they can operate independently over a vast area, as it's a big country. Decapitation strikes won't suffice to neutralize the threat. A prolonged shooting war with Iran is a disaster in the making for the US of I belligerents. They don't need nukes, that's obsolete. The June war showed they already got their deterrent, and now they've been augmented further by their friends.
It would be an utterly foolish move to go to war, but anything's possible when you got pedo cannibals calling the shots.