Wednesday's overwhelming House of Commons vote in favour of triggering article 50 of the Lisbon Treaty leaves Theresa May on course to start formal Brexit negotiations next month.
It will take a few weeks for the legislation to make its way through both houses of parliament but the prime minister could issue a formal notification of withdrawal from the EU at a summit in Brussels on March 9th.
The White Paper published on Thursday covers much of the same ground as May’s speech outlining her negotiating priorities at Lancaster House last month.
These priorities – controlling immigration, securing a free trade deal, limiting contributions to the EU budget and leaving the jurisdiction of the European Court of Justice – reflect the strongest motivating issues for the 52 per cent who voted for Brexit last year.
Security and defence co-operation warranted only a few paragraphs in the White Paper but one pro-Brexit Conservative backbencher suggested this week that it could become an important bargaining chip once the negotiations get under way.
“Britain’s security and defence contribution will be the elephant in the room during Brexit negotiations.
"The UK and France are the only countries with a significant military ready for deployment and, of these, the UK is the only country with troops on the borders of eastern Europe, " he said.
“If the negotiations go sour, then someone might say: ‘Why should we have troops deployed in Estonia?’ I think that would be a wrong reaction, but I am afraid I could lose that argument.”
US as ally
For the more romantic Brexiteers, leaving the EU represents an opportunity to restore what the prime minister calls “global Britain”, an outward-looking, freely trading nation with a global reach, economically, diplomatically and militarily.
For some, this means becoming an indispensable ally for Donald Trump’s America, even if that involves swallowing hard over unpalatable policies such as his travel ban on seven majority-Muslim countries.
Others dream of an alternative, Anglophone alliance with Canada, New Zealand and Australia, for which they have chosen the unlovely acronym Canzuk. Such an alliance would involve free movement of goods and people as well as co-operation on defence and intelligence.
More thoughtful defence analysts in Britain caution against such post-imperial pipe-dreams, urging instead a new “special relationship” on security and defence between Britain and the EU after Brexit.
Britain remains the most willing and able Nato ally in the EU, although its independent defence capability has diminished in recent years, so that all three branches of the armed forces are more operationally dependent than ever on the US.
Leaving the EU will mean leaving the Common Foreign and Security Policy and the European External Action Service, the EU’s fledgling diplomatic service.
And there are mutterings at Nato’s headquarters outside Brussels about stripping Britain of its automatic right to appoint the deputy supreme allied commander in Europe after Brexit.
The Balkans
The EU will continue to use instruments of soft power, such as energy and trade policy, to reinforce its security strategy, particularly in its neighbourhood in eastern Europe and the Balkans.
Britain could seek to replicate this strategy on a national level after Brexit, but budgetary constraints are likely to place a strict limit on such largesse.
In a new paper for the Royal United Services Institute, the defence think tank's deputy director, Malcolm Chalmers, warns against using Britain's "security surplus" as a bargaining chip in Brexit negotiations.
Instead, he suggests Britain should seek to persuade its EU counterparts that it is in both of their interests for security and foreign policy co-operation to survive, and even deepen, after Brexit.
It must balance its desire to use Brexit as an opportunity to become a more influential global power against its continuing interest in the security and stability of Europe.
“The UK’s departure from the EU is likely to deepen the recent trend towards a security policy focused on national interest.
The cumulative effect will be a foreign and security policy that is fundamentally different in emphasis than it was at the height of Blair/Brown internationalism in the decade after 1997.
Trump’s election – on a platform of “America First” – could further encourage this trend, throwing further doubt on whether the post-1945 western institutional order can now survive,” he says.