as John Lennon once sang. A very long time ago. I visited Gibraltar once, also rather a long time ago, and I remember that the Gibraltar government spokesperson continually referred to members of the delegation I was with as having "flown down" to Gibraltar - i.e not having gone in through Spain. One or two members of the delegation did in fact choose to enter Gibraltar through Spain, to demonstrate that they believed Gibraltar was part of Spain. But they could only do this because both Spain and the UK were, at the time (1998) members of the EU and thus obliged to keep their borders with other member states open. Before that, in the 1970s and 1980s, when the UK was a member of the EU but Spain had not yet joined, there was harassment by Spain at the border, with consequent difficulties for the Gibraltar economy. Before that, when neither the UK nor Spain had yet joined the EU, the border was blockaded and there was regular sabre-rattling by Spain. But at no time even then did Spain actually try to occupy Gibraltar militarily by force. They didn't dare.
As soon as the UK formally leaves the EU Spain can close its land border with Gibraltar and also blockade it by sea. The UK would then have to respond in some way, in the interest of the British people of Gibraltar, who have as we know chosen to remain British whenever their opinion has been asked. However, that does not mean war. One Spanish soldier's boot across the border, or one shot fired into Gibraltar from a Spanish gun, would however oblige the UK to retaliate on behalf of its citizens. As Michael Howard said yesterday, Theresa May as prime minister would have no choice. This is simply how it is. Spain knows this very well. Why has Spain never tried to take back Gibraltar (ceded to Britain in perpetuity by the Treaty of Utrecht) by force before? And what happened when another state occupied a British territory, 35 years ago this week? Some of us remember the Falklands.
I make no remark on the merits of all this, or indeed of the Treaty of Utrecht. Bismarck I think it was who coined the term Realpolitik.