The EU couldn't agree amongst themselves because the US (and its biggest vassal, the UK when it was in the EU) did everything to prevent such agreement.
We'll see what the States that were the most against any form of common European defense will do now that the US has proven unreliable. And if they are still under the delusion that the current US policies will go away, then it's time for Two-Speed Europe.
Don't blame this on the UK. UK leave vote was a few months before the 2016 election, so the timing is convenient. But let's not pretend that it was anything but complacency (that was shattered by Trump) is to blame here.