The crisis of parliamentary politics over Brexit is one of the biggest such crises ever.
There is ferment in the electorate at large and especially in the Labour Party on the issue. The outcome remains very open. At least some delay of Brexit beyond 29 March is likely.
Those facts set the frame for what the 9 March conference of "Labour for a Socialist Europe" can hope to achieve.
It can pull together and organise a campaign inside the Labour Party against Brexit for the coming weeks and months. So far "Labour for a Socialist Europe" has operated through a ramshackle committee of volunteers: the conference can elect a higher-paced committee.
"Labour for a Socialist Europe" has approached other anti-Brexit groups within Labour for cooperation, and has had success with "Open Labour", which will be helping to lead a workshop at the 9 March conference.
The campaign can then build on the victory registered by Labour coming out for a new public vote by pressing for the new public vote policy to be pushed effectively (not just as a designed-to-fail gambit) and for Labour to back Remain in a new public vote.
Shadow Chancellor John McDonnell has said that in the case of a new public vote, Labour would have to use its democratic procedures to decide which option to back (and he himself would back Remain). The best democratic procedure is a special conference. In any case, Labour needs a special conference to sort itself out.
"Labour for a Socialist Europe" should also have a campaign ready for an early general election (still possible if Theresa May just cannot find a Brexit formula which will pass Parliament) and for May Euro-elections if they happen in Britain (which, legally, they should if there is a long Brexit delay).
In recent months of "Brexit endgame", there has been a strong mood within Labour of wanting to defer to the leadership on the issue.
The leadership's shift on the new public vote, its visible disarray over whether to back Remain in that new public vote, and the approach of 29 March, make openings to shift that mood in the coming weeks.
The 9 March conference should gear the campaign to reach beyond pushing motions in Labour Parties and such.
It should promote on-the-streets campaigning and the creation or growth of local groups on the model of Nottingham's Left Against Brexit/ AEIP.
It should mobilise for a visible and militant L4SE bloc on 23 March and on other big national demonstrations.
It should stimulate activists to protest outside Parliament on 12 March and at other crucial votes.
"Labour for a Socialist Europe" should have its own placards, with messages like: Socialist Europe; Labour - back Remain; Defend and extend free movement - stop the Immigration Bill; Labour: call special conference.
Another wing of the campaign's activity should be face-to-face "singlejack" activity in workplaces, talking with workmates one-by-one with the "Labour for a Socialist Europe" petition (which will be revised to take into account Labour's shift on a new public vote).
The campaign should also organise to take the issues into the unions. There are difficulties there, because union procedures move slowly and time is short. But there are possibilities. For example, the Fire Brigades Union shows signs of new attention to the Brexit issue.
Longer-term, even if the government gets some sort of withdrawal agreement through Parliament, there will be much to do.
The withdrawal agreement, fundamentally, only states that Britain remains within EU rules until the end of 2020 (at least), agrees Britain's payments into the EU budget during that transition time, and fixes the Irish border "backstop".
Everything else - trade agreements, cooperation with various EU agencies, alignment or non-alignment with EU social and environmental provisions, immigration policy - will remain to be decided, within the vague parameters of the Tories' "future relations" document. What Labour says about all that - or about reversing Brexit even after a withdrawal agreement, which remains possible - has yet to be decided.
Already "Labour for a Socialist Europe" has been working with the Labour Campaign for Free Movement to oppose the Immigration Bill and support defence and extension of free movement.
It should seek also to make cross-Europe links with left and internationalist groups in other labour movements, and to push Labour on cross-Europe links.