The left needs to turn resolutely to Europe. It needs to pluralise (and not reduce) the sites of political conflict. It needs to build a pan-European movement through transnational party lists, shared political manifestos, and common protest initiatives. It needs to mobilise migrant workers rather than alienate them even further. It needs to campaign, in a coordinated way, not for a liberal superstate with a common army but for a European socialist federation which renounces neo-imperial ambitions once and for all. It needs to advocate neither the abandonment nor the reform of the European Union but a review of the Lisbon treaty that dismantles neoliberalism and bureaucracy. It needs to campaign for non-territorial citizenship, European-wide public ownership, extensive popular control of the economy, a new digital common, direct democracy, a federal parliament with revocable public offices and a non-technocratic, accountable, administrative apparatus based on strong principles of subsidiarity.
That's a great paragraph, but fighting for democratic-socialism in one nation-state is not harder than fighting for it transnationally. The only reason leftists called for Brexit is because Germany won't democratize the EU. Yes, it would be hard to get the UK to truly democratize but may well be easier than turning Germany from within the EU. Think of poor Greece, was it better off staying in? It was not!