ROME, April 18 -- Mild-mannered and measured no longer: US President Donald Trump's threat to destroy Iran's centuries-old civilisation was the red line that prompted Pope Leo to show his colours, analysts say.

Leo XIV, the first US pope, had been largely leaving criticism of the Trump administration - from its warmongering to crackdown on immigrants - to US bishops.

Elected in May, "from the very start he absolutely did not want to appear as the international anti-Trump personality", Vatican expert Marco Politi told AFP.

Pontiffs traditionally do not interfere in politics.

Leo was also wary of worsening a rift between conservative and reform-minded Catholics that dogged his predecessor Pope Francis.

But the spat may actually help re...