Pergamon is in Turkey. The ancient Greek city sat on a steep hilltop in what is now the Bergama district of Izmir Province, founded around 281 BC and reaching its peak under the Attalid kings in the second century BC. The Pergamon Altar, built between 180 and 160 BC and probably dedicated to Zeus, was 35 metres wide with a 113-metre frieze depicting the Gigantomachy — the war of the Olympian gods against the giants. Around 1878 a German engineer named Carl Humann, working on Ottoman railroad surveys near Bergama, noticed marble fragments being burned for lime. He alerted Berlin. Over the next decade German archaeologists negotiated with Ottoman authorities to remove the surviving marble — the Ottoman state was running on debt and granted excavation permits in exchange for share-of-finds arrangements that, today, are widely considered exploitative. The altar was reassembled in Berlin. Turkey has been asking for it back since the founding of the Republic in 1923.

Around the corner from the altar — when the museum is open — stands the Ishtar Gate of Babylon, glazed brick the colour of a deep summer sea, lions and dragons and aurochs picked out in cream and yellow. Built around 575 BC by Nebuchadnezzar II as the eighth gate of the inner city of Babylon, it was excavated by Robert Koldewey between 1902 and 1914 in what is now Iraq. Koldewey shipped the brickwork to Berlin in fragments — about half the gate, plus 30 metres of the Processional Way — under permits negotiated with the Ottoman authorities. The full original gate was much larger; what stands in Berlin is one of two registers and a partial reconstruction. Iraq has formally requested the return of the gate. Egypt wants Nefertiti from the Neues Museum next door. Turkey wants the Pergamon Altar. Greece wants pieces of the Antikensammlung. The collective restitution case against the Berlin museums is, depending on how you count, the largest open file in twenty-first century museum law.

The current Pergamon was not the first attempt. By 1904 the older Kaiser-Friedrich-Museum, today the Bode, was overflowing. Wilhelm von Bode, its director, commissioned Alfred Messel to design a much larger building specifically to house architectural-scale objects — gates, altars, facades — that no conventional museum could accommodate. Messel began work in 1906 and died in 1909; his friend Ludwig Hoffmann took over. Construction began in 1910 and ran through the First World War, the Spanish Flu, the great inflation of the 1920s, and into 1930, when the museum finally opened. It was bombed in the air raids on Berlin in 1944 and 1945. The Soviet army stripped the museum of loose items in 1945; many were returned to East Germany in 1958, but a significant fraction remains in the Pushkin Museum in Moscow and the Hermitage in Saint Petersburg, frozen in place by Russian cultural property law that prohibits restitution.

The current renovation is the result of a 2000 architectural competition won by the Cologne architect Oswald Mathias Ungers. The project replaces the courtyard entrance with a fourth wing and connects four of Museum Island's five buildings via an underground Archaeological Promenade. Work began in 2014 with a partial closure. Costs nearly doubled to €477 million by 2016 — partly because two old subsurface pump houses were discovered under the foundations, partly because the Berlin construction industry, recovering from the BER airport debacle, was charging more. Then the rest of the building closed in October 2023. While the museum is shut, the Asisi Panorama nearby offers a 360-degree painted reconstruction of the ancient city of Pergamon at its peak. It is the only place in Berlin you can currently get close to the Altar. It is, in some quiet way, the conversation the museum and Turkey have been deferring for over a century — a city the altar belongs to, rendered in paint.