The UK leaves the European Union at 11pm on Friday night, bringing to an end almost 50 years of political and economic ties.
What effect will the departure have on Britain's defence?
Professor Michael Clarke, a Distinguished Fellow at think tank, THE Royal United Services Institute, told Forces News he believes changes felt by the military will not be immediate.
"The Armed Forces won't directly feel the change as a result of anything that happens tonight," he said.
"But the longer-term implications are pretty big because this is a big strategic change in Britain's orientation towards the rest of the world and towards our European partners."
Professor Clarke said the defence review announced by the government is likely to look at "new technologies, new research projects, at integrating defence and foreign affairs and overseas aid and intelligence much more than in the past".
This, he estimates, could be concluded by November, or indeed early 2021, at which point future changes to the Armed Forces will become clearer "if the review does what it is promising to do."
However will Brexit bring a push towards a bigger EU army?
Professor Clarke does not believe so, saying "43% of all military research and development is British" and that "on its own the European Union is not going anywhere in the defence field".
"The EU will never be capable, in present circumstances, of doing more than deploying a battle group for peace support operations," he said.
"That's the most they can achieve on their own."
According to Professor Clarke, NATO will be "unaffected by Britain leaving the European Union" in a military sense, but he says Brexit may bring about "downstream issues whereby the political atmosphere of Europe gets worse", impacting our broader security.
"It may be more difficult, for instance, to maintain sanctions against Russia," he said.
"It may be more difficult to maintain a united front against Chinese pressures, as China becomes more integrated in the economies of Europe, and it certainly may be more difficult to hold the line on regulated free trade."
Professor Clarke said a big area in which there are no winners, is policing.