This is the flag of the Houthis [1], they are sponsored by Iran, as are the likes of Hezbollah, and Hamas. They have similar language in their charters.
That slogan has been around since 2002. The U.S. carried out a targeted strike against a leader of Al-Qaeda in Yemen, an enemy of the Houthi movement. Using U.S. foreign policy to somehow excuse a flag calling for "death to America, death to Israel, curse on the Jews" is a moral failure.
How is that not casus belli?
1 - https://en.wikipedia.org/wiki/Sarkha