I don't know if this is exactly what you're looking for (since this is less of a collaborator and more of a responsible bystander type thing) but I have an example.
During the Holocaust, a lot of countries (including America and many of its allies) were aware of what was going on for quite some time (death camps, oppression, genocide, etc.) before they joined the war, and didn't make any efforts to help or rescue the people in prison camps. Specifically, US officials received reports on the situation and actively resisted helping in any way, even in ways that wouldn't involve them going to war. They, and other equally informed countries, turned away boats of Jewish refugees, mainly children, who were eventually forced to return to Nazi occupied countries when not a single country would accept them and likely murdered or placed in camps as a result. Even after they had joined the war, the US refused to help those who were dying in camps. There were instances in which US planes, boats and troops were less than two miles away from death camps such as Auschwitz and could have easily saved thousands of lives with minimal damages to their own forces. But they continued to refuse, saying that they couldn't afford the distraction and that the best way to help those suffering was to win the war outright. Partly as a result of their cruel refusal to intervene, millions of people were slaughtered as the Nazis carried out their genocide of the Jews, as well as the Romani, homosexual, and disabled peoples of Europe (and a number of other groups I can't remember off the top of my head). This utter moral failure remains the topic of discussion and debate.
As a general answer to your question: inaction (political and military), not sufficiently pressuring the perpetrators to stop, not making the events center stage news to foster sympathy/empathy for victims, closing immigration for refugees, fostering feelings of fear and hatred towards victims w/in own country, etc
