They were a big deal in a handful of American cities, but at their peak they only few thousand members. I don't think you can characterize them as a threat to the U.S., and that was never their goal, their goal was self defense, freedom from racist systems that existed in America and education for the black community that they felt was being terrorized by the establishment. Here is their 10 Point Program that explains what they wanted:

1. We want freedom. We want power to determine the destiny of our Black Community.
2. We want full employment for our people.
3. We want an end to the robbery by the white men of our Black Community.
4. We want decent housing, fit for shelter of human beings.
5. We want education for our people that exposes the true nature of this decadent American society. We want education that teaches us our true history and our role in the present day society.
6. We want all Black men to be exempt from military service.
7. We want an immediate end to POLICE BRUTALITY and MURDER of Black people.
8. We want freedom for all Black men held in federal, state, county and city prisons and jails.
9. We want all Black people when brought to trial to be tried in court by a jury of their peer group or people from their Black Communities, as defined by the Constitution of the United States.
10. We want land, bread, housing, education, clothing, justice and peace.

The Panther's demanded their Constitutional rights. They were not trying to overthrow the system. They exercised their Constitutional right to bear arms and enraged police forces by stopping to observe arrests of blacks to make sure that the police followed proper procedures. They went so far as to carry copies of the law to show police who challenged them that they were within their rights to do what were doing. This type of stuff and other things eventually lead to them being illegally harassed, spied upon and in some cases assassinated by the FBI under J. Edgar Hoover.

The greatest impact that the Black Panther Party had on the civil rights movement was to provide what the establishment saw as a scary militant, alternative to the non-violent tactics of people like Martin Luther King Jr.

Despite their sometimes Marxist rhetoric, the Black Panthers were as American as apple pie and were willing to use violence to get the rights that they were promised under the Constitution.

I highly recommend David Hillard's book "This Side of Glory: The Autobiography of David Hilliard and the Story of the Black Panther Party" if you want to understand the motivations and aims of the Black Panther Party.