To add fuel to the fire is to make an angry situation worse. That letter only added fuel to the fire.
It describes something done into an argument already burning, so there must be a fire first. Add fuel to the flames is the same phrase with a different noun.
1phraseadd fuel to the fireTo make an angry situation worse.
“Naming him in the statement only served to add fuel to the fire.”
add fuel to the fire is a phrase.
Practised in Idioms