I'm sure you know there are a couple of letters of Imam Ali(as) to Muawiya namely letter 6, which says: Verily, those who swore allegiance to Abu Bakr, `Umar and `Uthman have sworn allegiance 1 to me on the same basis on which they swore allegiance to them. (On this basis) he who was present has no choice (to consider), and he who was absent has no right to reject; and consultation is confined to the muhajirun and the ansar. If they agree on an individual and take him to be Caliph it will be deemed to mean Allah's pleasure.

I am quite confused here, because Imam Ali(as) says that "It was deemed to be Allah's pleasure"

So I wanted to ask if these letters are Authentic at all(I know there is no isnad but still can be Authenticated)

If they are then how would explain them?

Also I know there is the sermon of Shaqshaqiya, so would that sermon supposedly contridict what he(as) has said here?

1- In this letter, Imam Ali (a) is refuting Mu’awiya’s claims and leaving no excuses for him. First of all, Mu’awiya recognizes the caliphate of those 3, so the Imam is telling him what’s different about me? If they were legitimate rulers them so am I, so on what basis do you fight me and reject my authority?

2- Secondly, Imam Ali (a) indicates that the Muhajirin and Ansaar unanimously pledged allegiance to him, and Allah is pleased with the person who unanimously was chosen. Now on the outer surface this statement could be seen as supporting Abu Bakr, but in reality it isn’t because there was no unanimous allegiance to Abu Bakr. Imam Ali refused to pledge him allegiance, along with a few others. So the statement of the Imam that the one who is unanimously chosen is accepted by Allah does not apply to Abu Bakr.

So the point of this letter was just to refute Mu’awiya even based on his own standards:

1- He believed those 3 were legitimate, so then why isn’t Imam Ali’s caliphate legitimate? What’s the difference?

2- Mu’awiya may try to argue that Muslims unanimously chose Abu Bakr (which isn’t the case of course), and so the Imam is telling him: Muslims of Medina unanimously chose me (so why don’t you obey me?)