For most of the occupation, southern Greece lay under Italian control, and the regime in the camps was comparatively lax. When the first political prisoners arrived at Haidari in early September 1943 - 600 people sent up from Larissa, including 243 communists who had already been jailed since before the war under the Metaxas dictatorship - they could still receive visits and letters, and were not forced to labour. That mercy lasted barely a week. Italy surrendered to the Allies on 8 September, and on 10 September the Germans took over the camp. The SS imposed a far harsher order: prisoners confined to their rooms, visits cut to once a month, and a regime of fear deliberately maintained.

One commandant casts the longest shadow. Paul Radomski, an early Nazi Party member, had run the Syrets camp near Kiev with a terror so brutal that even fellow SS officers recoiled; his own file called him "primitive." An eyewitness, Constantine Vatikiotis, arrested in October 1943, later described Radomski personally shooting a Jewish prisoner named Levy in front of the assembled inmates, supposedly for an escape attempt on the day of his arrest. The killing was a message, calculated to keep everyone in constant fear of their lives. Vatikiotis estimated that 2,000 people were executed during the months he spent there. Radomski's successor, Karl Fischer, traded open brutality for a quieter cruelty: a web of informants and spies planted among the prisoners themselves.

The single act for which Haidari is most remembered came on May Day, 1944. After ELAS partisans ambushed and killed the German general Franz Krech in the southern Peloponnese, the occupiers chose 200 communist prisoners from Haidari and shot them at the Kaisariani rifle range on 1 May. Reprisal killing on this scale was not exceptional in those months - through the spring and summer of 1944 the Germans swept Athens with mass arrests and raids, and the camp's population swelled to several thousand. But the May Day 200 became a symbol. They had been singled out, named on a list, and marched out to die for the act of partisans they had never met. Survivors said many of them faced the firing squad singing.

The Germans had already deported the Jews of Thessaloniki in 1943, but the communities of the formerly Italian zone were left untouched until the spring of 1944. The first Jewish prisoners reached Haidari in December 1943, held apart in the basement of Block 3. Then, in late March, the machinery of deportation closed in everywhere at once. On 23 March, between 700 and 1,000 Jews of Athens were rounded up and brought here; days later came 614 more from Epirus and western Greece, among them families who had carried foreign passports they believed would protect them. Haidari was the funnel. Almost all of those deported from the old Italian zone were murdered within days of reaching Auschwitz-Birkenau in April 1944.

For decades after the war, Haidari was deliberately forgotten. The camp had become bound up with the memory of the executed communists, and after the Left's defeat in the Greek Civil War, public commemoration of its role in the resistance was suppressed. The site remained a working army base, off limits to mourners and historians alike. Only in the 1980s, with the rise of the socialist PASOK government and new laws recognising the resistance and pursuing national reconciliation, were the gates finally opened for annual remembrance. Block 15, where so many were held, is now a declared national monument; it even appears in the emblem of the Haidari municipality. The people who survived return there still, standing before the same walls.