Why use nerve gas in a war he is winning? Who gains from US-Russia war?

More than 80 people were killed in a suspected chemical attack on Khan Sheikhoun, a town in Syria. Local people, who are rebels fighting the government of Bashar al-Assad, and Western nations immediately blamed bombing by government warplanes. The Syrian government, backed by Russia, said that its planes had hit a rebel ammunition dump where the rebels were storing chemical weapons. In situations of this sort what the US says is accepted as gospel truth while it blithely continues to use white phosphorus in Iraq. After lying strenuously the US was forced to admit a massacre of civilians using white phosphorus in Fallujah. Depleted uranium will cause birth defects in Iraq for centuries to come. That is not to condone using sarin against civilians but the US punished Assad by launching 59 Tomahawk Cruise missiles at Shayrat airbase near Homs, from where the planes were supposed to have taken off. At $1 million per missile this was a pretty expensive fireworks display. Replacing them will surely cost more because prices must have gone up. Did the strikes work? Apparently government planes were bombing the same town, taking off from the airport the next day. Russia condemned the attack as an act of aggression against a sovereign state. So what is going on in Syria? It started in 2011 with protests against the Assad government, inspired by protests in Tunisia and Egypt, which were known as Arab Spring. Hosni Mubarak was deposed in Egypt, Gaddafi was killed in Libya and Zine El Abidine Ben Ali of Tunisia sought refuge in Saudi Arabia. Assad was having none of it and responded ferociously against the protesters. He was advised by Iran which crushed its own protests brutally in 2009 when Mahmoud Ahmadinejad was alleged to have stolen the election for president. The opposition leaders, who supposedly won the election, are still under house arrest. Nobody would dare to meddle directly with Iran but in Syria Gulf states, Turkey and the US provided money and arms to the rebels while Iran helped Assad by sending in Hezbollah, a Shia militia, from adjoining Lebanon and Russia helped by bombing the rebels. The mystery is why the Western nations did not kill Assad as they killed Muammar Gaddafi in Libya? It maybe because they needed Iran's help and Shia militia to fight ISIS in Iraq. In 2014, 30,000 Iraqi soldiers guarding Mosul ran away from 1,500 ISIS fighters, complaining of lack of air cover. They seem to have made a habit of running away even though they are heavily armed. Why would Assad use chemical weapons in a war he is winning, when it is sure to invite reprisals? The glib Western answer is that he feels he is invincible, which is rubbish because Assad is not an imbecile. Did some other country release nerve gas under cover of bombing? Who gains from a war between the US and Russia? Interesting.