His grim warning came as fighting prevented chemical weapons inspectors from visiting two sites, although UN chief Ban Ki-moon said the mission to destroy Syria's arsenal by mid-2014 was still on track.
Brahimi has been seeking to build on the momentum of last month's US-Russian deal to eradicate Syria's chemical weapons in order to launch the so-called Geneva II peace talks proposed for next month.
But the talks have been cast into doubt by the increasingly divided opposition's refusal to attend unless President Bashar al-Assad agrees to step down, a demand rejected by Damascus.
"What history teaches us is that after a crisis like this there is no going back," the Algerian diplomat told the Jeune Afrique website ahead of his first visit to Syria since December, when he angered the regime by insisting that all powers be handed over to a transitional government.
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The veteran troubleshooter admitted "the entire world will not be present" at the talks, but said the alternative to a political settlement could be a failed state in the heart of the Middle East.
More than 115,000 people are estimated to have been killed in Syria's 31-month conflict, which erupted after the regime launched a brutal crackdown on peaceful pro-democracy protests.
In the latest blow to peace efforts, 19 Islamist rebel groups issued a statement Sunday saying anyone who attends the Geneva talks would be committing "treason" and "would have to answer for it before our courts," implying they could face execution.
Russia denounced the warnings, saying it was "outrageous that some of these extremist, terrorist organisations fighting government forces in Syria are starting to make threats."