The use of napalm against civilians or in areas with dense civilian populations is against international law starting in 1980.
https://en.wikipedia.org/wiki/Protocol_on_Incendiary_Weapons
Strictly speaking Assad did not break the law because Syria never signed the treaty.
Strictly speaking???
More like as wide a an 18 wheeler speaking...
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strictly speaking the civilians weren’t incinerated, as there were bodies to collect.