The former head of the Stop the War coalition has promised mass protests if the "deep state" tries to thwart Jeremy Corbyn's foreign policy agenda.
Andrew Murray has previously warned that unelected forces are out to undermine him and the Labour leader.
He told a fringe meeting at the Labour conference in Liverpool that Mr Corbyn would lead an "anti-war government".
Mr Murray, a part-time adviser to Mr Corbyn, is chief of staff at the Unite trade union.
He told a Stop the War meeting that the "war on terror" had failed and "negotiated peaceful settlements" were needed to end conflicts in place like Afghanistan and Libya.
Mr Murray also called on Mr Corbyn to end the "special relationship" with the US, saying that President Donald Trump did not care about the UK and probably thought of it as a "golf course with a Queen attached".
When asked what he would do if the "deep state" tried to block Mr Corbyn's foreign policy aims, Mr Murray said: "We would rely on the mobilisation of the mass of people as we did in 2003."
Hundreds of thousands of people took to the streets of London in February 2003 to voice their opposition to military action against Iraq. Mr Corbyn, then a backbench MP, addressed the crowds in Hyde Park.
"In my view, Jeremy Corbyn stands on the threshold of Downing Street today because he stood on that stage in 2003 and spoke for the millions of all of us who wanted an agenda of peace and justice - not neo-Conservative war - and we are now within touching distance of that agenda coming to fruition," said Mr Murray.
The trade union official, a former communist, was last week mocked by Labour's deputy leader for suggesting the "deep state" was against him.
The packed Stop the War fringe meeting was also addressed by shadow cabinet members Richard Burgon and Diane Abbott, as well as leading Corbyn-supporting MP Chris Williamson, among others.
Mr Burgon accused some of the party's MPs of being "unwavering and very vocal advocates of bombing".
Ms Abbott said Mr Corbyn's "internationalist" policy agenda was just as important as his domestic one - and "Jeremy Corbyn in Number 10" would not just make people richer it would make them "safer by being a force for peace".