Ayatollah Fadlallah opposed the concept of Velayat-e-faqih, an Iranian invention which gives unchallenged authority in politics and theology to the Supreme Leader - currently Ali Khamenei. Iran, meanwhile, never recognised Ayatollah Fadlallah as a marjaa'.
So while Ayatollah Fadlallah did not hold an official position and cannot be replaced in the same way that a judge or minister would be, Iran will likely seek to promote its own favourite to lead Lebanon's Shias.
"Admittedly, US policymakers have typically not been players in the arcane world of Shia clerical politics," wrote David Schenker from the Washington Institute for Near Eastern Policy.
"How ironic, though, that Fadlallah - a man who Washington labelled a terrorist in 1995 - stood as the last bulwark against near total Iranian hegemony in Lebanon."
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