Why should you believe anything American and French government says?

American government has quite a record of manufacturing evidence to further their own agenda. Iraq WMD is just the most famous example, because every single politician of importance was parroting it for a year, so it stuck with people.

You're starting from a position of American government doesn't lie therefore anyone with a different opinion is lying.

Russian government lies, American government lies, Assad is a dictator, a lot of rebels aren't much better and many are worse. In that chaos, somebody gassed civilians. How can you be so sure it was Assad?