The US declared war on Romania on 5 June 1942, a week before the first American attack on Ploieşti (the Halverson Project or HALPRO) was launched.

After HALPRO, the Germans beefed up the anti-aircraft defenses around Ploieşti. Luftwaffe General Alfred Gerstenberg was responsible for the defense of the oil fields. Gerstenberg was highly competent and put together an excellent defensive network consisting of anti-aircraft guns and balloons as well as coverage by both German and Romanian fighter aircraft. By the time the Americans launched Operation Tidal Wave on 1 August 1943, Ploieşti was one of the best-defended targets in occupied Europe. If you’d like to read about Gerstenberg’s defenses in detail, I recommend James Dugan and Carroll Stewart’s Ploesti: The Great Air-Ground Battle of 1 August 1943. It’s also discussed in Jay Stout’s Fortress Ploesti.

As you probably already know, Gerstenberg’s setup held up very well during Operation Tidal Wave and the Americans suffered heavy losses in men and aircraft. Most of the refineries were repaired relatively quickly and Tidal Wave was both an operational and a strategic failure.

The Romanian General Staff commissioned a study from the Gendarmerie about the attack and also prepared their own evaluation, including a recreation of the Americans’ attack routes so that they would be better prepared to intercept them the next time (although there wasn’t really a next time, since by the time the Americans launched their next attacks in spring 1944, they were able to use captured airfields in Italy instead of having to fly from Libya). There’s a copy of this report (in Romanian) in NARA RG 242.

The Romanians also had to open their first POW camps for Anglo-American prisoners after Tidal Wave. They issued new instructions for the handling of Anglo-American prisoners and accounting for downed airmen on 13 October 1943 (also in NARA RG 242). They only held around a thousand Anglo-American airmen and the conditions in these camps were relatively good (the prisoners called them the “gilded cage”). I’m researching Romanian POW camps at the moment so I’m really glad you asked about this. If you have any more questions please ask away.