While my sympathy is strongly with the Roma, Jews, Slavs, homosexuals, and disabled people that the Nazis went after, and I would never spit on their graves, your statement (and implied accusation) is something of a stretch, if not entirely wrong.
While the Nazis began to persecute the Romani the moment they rose to power, and codified some of that into the Nuremberg Laws in 1936, and moved some Romani into camps before the war, they didn’t begin a formal program of extermination until after the US had entered the war:
[QUOTE=Wikipedia]
On December 16, 1942, Himmler ordered that the Romani candidates for extermination should be deported to Auschwitz-Birkenau. To the Romani people of Europe, this order was equivalent to the January 20 decision of that same year, made at the Wannsee Conference, at which Nazi bureaucrats decided on the “Final Solution” to the “Jewish problem”. Himmler then ordered, on November 15, 1943, that Romanies and “part-Romanies” were to be put “on the same level as Jews and placed in concentration camps.”[56]
[/QUOTE]
From here.
Nazi Germany declared war on the US on Dec 11, 1941 – more than a year earlier. In fact, over the course of the year before the Nazi death order against the Romani, the follwing notable events took place:
January 13, 1942: German U-Boat offensive against the United States (Operation Paukenschlag) (Americans were dying in naval war against Nazi Germany).
August 17, 1942: First all-American air attack in Europe (Americans were dying in air war against Nazi Germany).
November 8, 1942: Operation Torch, the British-American invasion of North Africa (Americans were dying in ground war against Nazi Germany).
Now, it is true that einsatzgruppen followed the German armies into the Soviet Union’s territories beginning in late June, 1941 – six months before Germany and America were at war – and they definitely killed Romani civilians (among many other classes of “undesirables”). At that time, those killings were in conquered territory, and associated with the advancing armies, and to some extent represent targets of opportunity – it was easy to get away with killing in the chaos of the titanic ground war. While those killings were certainly part and parcel of the Nazi attack on “undesirables” and a part of the overall genocide, the formal orders to begin extermination camps for “undesirables” in all German territories came about 18 months later, after the Americans had been dying at Nazi hands for over a year.
The American record on the Holocaust and the Porajmos is not a proud one, to be sure, a mixed record of slow reaction and a degree of willful blindness, but we were not “waiting out the second world war” while it happened.