On February 15th, 2003, the world saw the biggest protests ever recorded. In cities around the globe, millions took to the streets to send a message to the US and British governments: do not go to war with Iraq. The enormous strength of opposition convinced many that they would win the argument - how could a democratically elected politician ignore such an outcry?
Yet little more than a month later, Iraq was invaded. The protesters' views were cast aside. And with them went many people's faith in our systems of democracy and in the people's ability to influence those in power. Over 12 years later, for some, their hope still hasn't been recovered.
And yet this Saturday I will join more than 50,000 optimists in taking to the streets of London once more - this time to stand united against austerity. I will join hundreds of Green Party activists from up and down the UK in the Green Bloc, making our call for investment to create a fair and sustainable economy heard.
I know that one march won't convince George Osborne to instantly change track. But it's the cumulative effect of all our marches, demos, and protests that is so powerful - each sending out the message that an alternative future is possible and that there is mass support ready to deliver it.
Battles for change have never been won overnight - but they have been won. Women won the right to vote. Slavery was brought to an end. Rights that were once thought beyond our reach have been delivered. The Westminster vote against bombing Syria in 2013 was in part a delayed response, a taking on board, of the Iraq marches.
Together we can and will deliver change.
Saturday is a major step along that path. It's not often that 50,000 likeminded people from across the country come together, all prepared to take action to save what is left of our public services. We must use this march to make new friends, to share our ideas, to remind each other that we are not alone.
It is vital that we see this demonstration as just the beginning. In 2003 people thought one amazing day of demonstrations was enough to make a change. The lesson we must learn is that continuing pressure, continuing determination, is key.
We won't be going home on Saturday thinking our work is done. The next step is to use our energy and come together with our new friends to plan more marches, more petitions, new ways of making life difficult for those who plan to cut services we rely on and take us in the wrong direction on environmental policy.
We've already seen the beginnings of this since the election - an upsurge of activism that I've seen on anti-austerity marches from Swansea to Sheffield that have brought together campaigners on issues ranging from excessive testing of children in schools and opposition to fracking, to the fight to protect the NHS against privatisation.
We must leave the march on Saturday and spread the word. If everyone who attends can persuade just one more person that the government's programme of spending cuts is unnecessary and unfair, we can begin to build a real movement.
Already, we can say that the vast majority of people in this country did not vote for the Conservatives' vision of a state with no safety net for those who fall on hard times that talks about dealing with climate change and protecting the natural world while actually doing the opposite.
But our efforts will have really paid off if the next general election centres not on how much government spending should be cut, but on whether or not it should be cut at all, and how we should be investing in an economically, socially and environmentally sustainable future.
Only then, when we have successfully cracked public debate open to considering alternatives can we remove the likes of Cameron and Osborne from power. With time, and through collective action, we can end austerity and take positive steps towards investing in our communities.