Throughout most of the middle of the 19th Century, the UK was able to stand alone, as there was a rough balance of power on the continent, and the strength of its navy was unrivaled. This was referred to as “splendid isolation.” The rise of Germany and the growth of its navy after 1870, however, upset the balance of power and made the UK much more vulnerable. Upon belatedly discovering that it had no allies on which to rely for help with colonial disputes in particular, the British government was compelled to edge towards, and ultimately join, the alliance that became known as the Triple Entente.
You might well say that conditions are different now, because, even after Brexit, the UK would be protected by NATO and its own nuclear weapons, and that there is no military threat analogous to that of the German army and navy. I would agree with that, but the present state of affairs promises its own difficulties. The UK may well break up if Brexit passes. Trade with Europe figures to become substantially more difficult, as will relations with the Republic of Ireland. And the UK’s leverage on trade issues with the unfriendly nations of Russia and China will be much diminished. All this to keep out a few Polish plumbers. Is it really worth it?
“Splendid” isolation, indeed.