To renege on something, you decide not to do something that you promised to do. In the cartoon, Babs states that we reneged on every single treaty. In other words, she’s accusing earlier Americans of not honoring the treaties they signed with the native peoples.
As a result, Babs states that Native American culture was decimated and that they were forced into poverty onto squalid reservations. If something is squalid, we say that it is dirty and unpleasant. It can also imply unpleasant conditions because it involves dishonest, illegal or immoral behavior. Many people who live in government housing projects (les cités) live in squalid living conditions.