Out of curiosity and having not done so before (and mildly inspired by current events) I decided to read the US Constitution. I'm starting with the amendments because my son is learning about them in school so it's fun to learn together. The 25th amendment determines that the Vice President will become president after the death of the President. The amendment, however, was ratified in 1967, which is after the Kennedy assassination. I always was under the impression that this was a fact always and didn't realize it was so late that the amendment was made. I was just wondering why it seems to have always just been the case anyway to have the VP take over.
*Sorry for the edit, my dog jumped on me and posted before I had finished typing
So this is a great question, and provably something that would have made more sense if you read the text of the Constitution first. This site has the full text, including the paragraph in Article II of the Constitution that originally provided for the succession:
In Case of the Removal of the President from Office, or of his Death, Resignation, or Inability to discharge the Powers and Duties of the said Office, the Same shall devolve on the Vice President, and the Congress may by Law provide for the Case of Removal, Death, Resignation or Inability, both of the President and Vice President, declaring what Officer shall then act as President, and such Officer shall act accordingly, until the Disability be removed, or a President shall be elected.
The issue with that is that it's not clear whether the VP actually becomes president, or some sort of acting presidency happens; also, there's some lack of clarity of how the Congress would deal with the "inability" of the President/VP to carry out their duties. This kind of becomes a big deal after WWII, when the president is the person in charge of the nation's nuclear arsenal, and what happens when he/she is unable to carry out duties.
This issue came to a head during Eisenhower's presidency, when he suffered a heart attack and then intestinal problems requiring surgery -- VP Nixon presided over Cabinet meetings but there was no formal transfer of power, and ambiguity regarding control of nukes is not something we want.
So the amendment basically does four things:
clarifies that the VP becomes president when that office is vacated;
provides for choosing a new VP when that office is vacant;
allows the president to delegate powers if they are ill/unable to discharge duties;
allows the VP and cabinet a process where the VP can assume presidential powers if the president is incapacitated and has not done no. 3 above.