According to some Hadith book written by some scholar,During the Battle Of Karbala, Salma went to visit Muhammad's wife Umme Salamah. In the Convesation between them,Salama told she had seen Allah's Messenger with dust on his head and beard. She asked him what was the matter and he replied, `I have just been present at the slaying of al-Husayn.' Does such thing really happened ? If Muhammad present in that battle field why he didn't save Al-Hussain ?
First, the significance of this is to prove that the prophet actually have seen his grandson in the battle of Karbala, this means that he knew what happened to him, who killed him and everything happened there.
Now, your problem is that you feel that Al Hussain was actually in trouble to be saved. Shia believe that Al-Housain was able to survive but he didn't he chose to sacrifice his life to save Islam. please read more about battle of Karbala .