Al Shabab is a organized branch of militants who are very anti American, I don't think they pose much of a threat on US homeland but they do pose a threat to our operations in the Middle East, I think these organizations should be dealt with swiftly before they become enough of a threat to do attacks on the US homeland.
It really doesn't take much for us to leave ourselves wide open. The terrorists groups just have to find that opening, it would be tragic if they did attack us on a small scale,but that's all it would be. It would just be another issue in the long run on our views on terrorism.
Yes, Al Shabab is a threat to America, because they are both extreme and willing to do what they did to Kenya. This is the group that attacked a random mall in Kenya. They are extremists. If they hate Kenyans that much, they must really hate us. We must be wary of Al Shabab.
Al Ahabab does not pose any significant threat to America at this time. Perhaps they will in the future. By contrast, America is a threat to the whole world. If any other country in the world did what the United States does on a regular basis, the self-righteous cries of "terrorism" from Americans would be deafening.
I do not take any threats from Al Shabab seriously at all. In my mind, there really is no massive threat to America. We are engaged overseas and face problems overseas, but there is no country that would dare be stupid enough to invade the United States. Even single attacks are almost always thwarted.
I think the biggest threat to America at this moment is America. Everyone is against everyone, no one can agree on anything. People feel the need to fight about everything, even when it's not important. Until the country can come together as a whole it will continue to be the biggest problem and threat for itself.