A recap . . .

Duke William's, claim is from a promise made in 1051.

According to Duke William, King Edward the Confessor promised him the throne.  In the past, Normandy had looked after and helped King Edward's family, therefore it was the Confessors way of ‘repaying the debt’ and also solving the ‘succession (or rather ‘the lack of) problem’.

The ‘future’ King Harold was not happy.  He wanted to become England’s Monarch himself and didn't want any ‘Normans’ beating him to it (understandably so in my mind). Edward the confessor knew that Harold Godwinson didn't agree.

It is said by ‘Norman’ sources, that Edward forced him to go to Normandy in 1064.  Harold had to swear an oath to ‘support’ William's claim.

But ‘Saxon’ sources (The Anglo-Saxon Chronicle) , give a different reason for Harold going to Normandy! It was to rescue people taken prisoner.  According to the Saxon sources, the hostages were Hakon (Harold's nephew) and Wulf (Harold's brother). To me, that sounds more ‘plausible’ but that is merely ‘my own’ speculation on the events.

An ‘oath’ was, sworn – it is depicted in the ‘Bayeux Tapestry’, but exactly ‘what it was’, we are still uncertain.
For Duke William, the oath is very important.  It gave him another reason to enter England in 1066.  According to William - Harold broke his ‘oath’ - breaking his promise to God.

According to Duke William, Harold promised to support and recognise Duke William's claim to become King.

Harold may not have promised to do this - he may have been tricked!
And so, Edward the Confessor, popped his clogs . . .