Can Geneva solve the Syrian peace puzzle? The struggle to gather all the interested parties in Syria's civil war around the same table is like the old puzzle about how to get a goose, a fox and a sack of grain across a river. Invite Iran to the peace talks in Geneva on Wednesday, and the Syrian opposition will walk out, closely followed by Saudi Arabia. Shun Iran, and the outside power with greatest influence on events will be liberated from any obligation to accept whatever agreement is reached. Whether all the parties will actually attend the summit is still uncertain. But amid the diplomatic manoeuvring, some fundamental points are worth restating. First, common humanity dictates that all these countries should be able to find points of agreement, regardless of their differing interests. Would anyone deny that Bashar al-Assad's regime must allow aid to reach the areas sealed off by his forces? Equally, his use of barrel bombs - peculiarly horrible weapons packed with nails, petrol and high explosives - is surely unconscionable. Is it asking too much for everyone to agree on these propositions? The second truth is deeply uncomfortable for Syria's opposition. We may not like it, but Iran will have to be part of any settlement. Tehran has sent thousands of troops from its Revolutionary Guard to fight alongside Assad. He is now a leader kept in power by Iranian bayonets. As such, no agreement will stick without Iran's signature. Finally, all parties must beware of Assad's narrative that Syria's war has become a struggle between his regime and al-Qaeda - and that he is therefore the lesser of two evils. Assad has actively helped extremists win influence within the opposition, precisely to confront us with this choice. And that might serve as the most important lesson of all: that Assad is just as cynical as he is brutal.