Turkish President Recep Tayyip Erdoğan has once again knocked on Europe's door. Speaking at the opening of the NATO Parliamentary Summit in Istanbul, he seized the occasion to remind Europeans of what he has repeatedly called on them to do — namely, to admit his country as a member of the European Union. This time, however, he phrased it differently, calling for Turkey's integration into what he described as the European defence architecture.

He has pursued this ambition for a long time, and his efforts have not met with success on any occasion, yet he has never despaired. He has always believed he must keep trying and never stop, driven by an inner conviction that Turkey deserves to be a member of the Union — that its geographical proximity, which makes it almost an organic part of Europe, qualifies it for membership, as does the size and global ranking of its economy.

It is no secret that the Turkish economy is one of the G20 economies, and membership of that group means it is among the 20 largest economies in the world. Nor is it a secret that Turkey's entry into that group came under Erdoğan himself, which is why he continues to feel, deep down, that an economy of such standing cannot justifiably be rebuffed when it seeks to join the Union.

The Europeans have rejected the bid every time, always finding a ready pretext for refusal — and when they could not find one, they invented it — because what mattered above all was that Turkey should not find the door open at the Union's headquarters in Brussels.

Among the stated pretexts, for example, was the claim that there are specific criteria for joining the Union, and that the Union's leadership in the Belgian capital would consider Turkish membership with interest once it found those criteria to be fully met.

Erdoğan never took that at face value. He would hasten to work on the required criteria, and whenever he believed he had done enough, he would return to knock on the door — only to have it opened by someone informing him that what he had achieved was insufficient and that he must resume his efforts in the hope that things might change.

For some reason, as one follows Turkey's desire for membership and the rebuff that has awaited it — and continues to await it — from the European side, one senses that there are other reasons for the rejection beyond the stated ones. But Ankara has naturally had no recourse except to engage the Europeans on the declared reasons rather than the hidden ones, and they have not despaired; they have returned each time with renewed hope.

It was from this hope that Erdoğan issued his latest call to Europe as he opened the Parliamentary Summit in Istanbul at the end of last June — in a city that has always been described as one whose location places half of it geographically in Europe and the other half in Asia.

Turkey holds membership in the North Atlantic Alliance, which is headed by the United States of America, and the Turkish military ranks second among the armed forces of Alliance member states by number of personnel, while Turkey's defence industries rank 11th among their counterparts worldwide. And yet, despite all of this, Turkey finds itself standing outside the walls of the Union, knocking on a door that does not open — or, when it does, opens only to offer an apology for declining membership.

If there is a phrase that fits the Turkish president in this context, it is what the poet once said: that his resolve was firm, but fate refused.