'The post-mortem era of prosperity was due to all competition (UK, Japan, Germany) recovering from WW2.'
No because don't forget it was the US that saved all of these countries with the Marshall Plan. They were all broke and ruined. It was US money that regenerated them and US troops that provided protection for them.
'Once they were back on their feet the US was under pressure.'
The globalist began to ship jobs out of America as they built up the third world.
'Now China is the biggest manufacturer in the World'
As Trump says, what happened in China is the "biggest theft in the world" where the US built China with factories relocating there for cheaper labour and for allowing the huge trade deficits which then occurred.
'How will the US get back to no 1 when it is the world's biggest debtor nation?'
Because the world is going to have to come to a deal to help America out of this or America will eventually collapse and that may lead to a huge war. A deal has to be done and Trump wrote "The Art of the Deal", he can pull it off, whereas the globalists are just increasing America's deficit every year.