‘Power is power” is the send-off lament of this Brazilian documentary, which looks back at a nearly forgotten episode in the long preamble to the second Iraq war, of 2003-11, one of breathtaking cynicism. The film centres on Brazilian diplomat José Bustani, who in 1997 was made director general of the intergovernmental Organisation for the Prohibition of Chemical Weapons (OPCW). In the flush of 90s global village optimism, it was a utopian attempt to abolish this sadistic category of armaments once and for all.

After early success bringing many countries into the fold, Bustani attempted to get Saddam Hussein’s Iraq to accede to the organisation’s Chemical Weapons Convention. The OPCW knew that the first invasion had destroyed his chemical weapons production capability, and that any remaining stocks were past their sell-by date; letting in weapons inspectors benefited everyone, including ordinary Iraqis being squeezed by sanctions. Everyone but the George W Bush administration, that is. Needing a pretext for invading and not wanting Bustani to give Hussein a free pass, it began a campaign to oust him from the organisation.

As outlined here by figures including OPCW insiders, former Brazil president Fernando Henrique Cardoso and the current president, Lula da Silva, the story plays out like a real-life version of the George Clooney film Syriana, with an embattled idealist trying to resist overwhelming forces. The Americans bugged Bustani’s office, co-opted his own security chief against him and finally used their leverage, in the person of Bush’s bruiser John Bolton, to undermine previously favourable support within the OPCW. Realpolitik reigned.

When Tony Blair was later confronted by Bustani, then Brazil’s ambassador to Britain and still adamant Iraq had no WMD, Blair allegedly blushed and said: “I hope he’s not right.”

Bustani had enough status to survive the fallout – and now spends his retirement as a concert pianist. (Perhaps the orchestra we see him leading is a metaphor for the global concord he couldn’t secure.) But the US may have permanently damaged the OPCW’s credibility, with the organisation again becoming a political football in the furore around an alleged 2018 chemical weapons attack in Douma, Syria. This incident is even more opaque, and director José Joffily struggles to fully align it with his thesis. But with Russia trying to further circumvent the OPCW, this coolly outraged film shows how Washington’s unilateralism has been a gift to even more belligerent parties.

• A Symphony for a Common Man at Bertha DocHouse, London, from 15 September.

• This article was amended on 16 September 2023 to remove text that was inadvertently included due to an editing error.