This week Labour MPs are likely to have to cast what will be one of the most important parliamentary votes of their career when the Prime Minister brings his proposal for Britain to join the bombing of Syria to the Commons.
It will be vital for two reasons. The first and most important is that Labour once again risks being tarred as a party of illegal and irrational war.
The 2003 vote to invade Iraq has haunted Labour ever since - losing millions of votes that have yet to be regained and causing around half the party's membership to resign. Nothing in Labour's history has traumatised it quite so much perhaps.
Now again Labour is being asked to support a war that lacks clear United Nations sanction - or a coherent purpose.
The war the Tories are pushing will only add to civilian deaths in Syria, to the number of refugees fleeing the country. It will further feed the cycle of violence - terrorism/war/more terror/more war - that has brought so much suffering to the region and the world this century.
It has been argued that British missiles are now so sophisticated that they don't cause many civilian casualties as "collateral damage". What a sinister, cold, phrase that is - it is lawyer's language for saying where hundreds of innocents would have been killed previously, now it is merely a few dozen.
The Prime Minister has hardly made a persuasive case for Britain attacking the very people who would have benefitted from the bombing he wanted in Syria just two years ago.
Already the most powerful air forces in the world are bombing the Islamic State. Even were it the right thing to do, the RAF could add little to their so-far ineffective efforts.
Everybody agrees that IS can only be defeated by a ground army which can secure some support from the people of the region itself. Not finding one, the premier has simply invented a mythical 70,000-strong opposition army - his own version of Tony Blair's infamous "45 minute warning" over Iraq.
This has rightly drawn widespread disbelief - to the extent that this "army" exists at all, it is largely fighters aligned with either al-qaeda or the Turkish neo-fascist "Grey Wolves", who are hardly "moderate" and anyway will never turn against IS.
The truth is that there needs to be a peace agreement between the Assad regime and its opponents leading to a transitional administration which could then take on IS. For years, the Prime Minister has worked against such an agreement.
Former Tory MP and respected journalist Matthew Parris agrees. He wrote in The Times: "Jeremy Corbyn is right. Joining the bombing in Syria will do nobody any good. And the funny thing is, I think that in its heart Britain knows that."
The real reason for the bombing plan is more likely the view put by one general this week that it will secure Britain "a place at the table". A sort of macho "me too" approach to war.
I am sure most British people, never mind Labour supporters, will not regard getting Phillip Hammond a "seat at the table" as worth a single Syrian or British life.
But there is a second reason why Labour MPs vote this week will be so significant - a lesser reason in the big scheme of things, but vital for the Party's future.
It is because the issue of Syria is being used as the thin edge to stage a coup against Jeremy Corbyn, Labour's (overwhelmingly) elected leader.
He has been denounced for writing to MPs and party members making his views on Syria clear - as if his huge mandate, which included support for his long-standing anti-war record, had simply earned him the right to be seen but not heard.
Yet at the same time members of the Shadow Cabinet are making their own pro-bombing views plain, either publicly or in off-the-record briefings.
And backbench MPs are even calling on him to quit for having the temerity to maintain his values and principles, with one even comparing him disgracefully to a "fuhrer". That is not open debate, it is abuse and should have no place in the party.
The thought that some Labour MPs might be prepared to play intra-party politics over an issue such as this will sicken all decent people.
And they are playing with fire. Any attempt to force Labour's leader out through a Westminster Palace-coup will be resisted all the way by Unite and, I believe, most party members and affiliated unions.
I have made it clear that I think Jeremy Corbyn is still on a leadership learning-curve. I would like to see him lead with greater firmness and authority, not less. He has, goodness knows, the mandate to do so.
But Jeremy is where he is because, among other reasons, Labour people were sick of politics-as-usual: Politics that pander to the reactionary media, that backs wars with no regard to the national interest, and that shows contempt for the views and votes of ordinary people.
I do not know whether there will be a "free vote" on Syria or not. However, that is a decision for Jeremy to make.
But we cannot have a "free-for-all" party. If those Westminster bubble-dwellers who hanker back to the politics of the past cannot show the elected leader - and those who voted for him -more respect then they are writing their own political obituaries. So be it - but the price should not be paid in Syrian lives.
Len McCluskey is General Secretary of Unite the unionSuggest a correction