One example of "painful experience that freedom is never voluntarily given by the oppressor" in United States history is the Civil Rights Movement of the 1950s and 1960s. Despite decades of peaceful protests and civil disobedience, African Americans were still denied basic civil rights and faced widespread discrimination and segregation. It took sustained legal and nonviolent pressure, including marches, boycotts, and sit-ins, to force the government to take action and pass legislation to end segregation and discrimination.
An example of "justice too long delayed is justice denied" can be seen in the case of Dred Scott v. Sandford (1857) where the US Supreme Court ruled that African Americans, whether enslaved or free, could not be American citizens and therefore had no standing to sue in federal court. This decision upheld the institution of slavery and denied basic rights to African Americans, further delaying justice and equality for decades until the Civil War and the 13th, 14th and 15th amendments to the US Constitution.