There once was a time when nationalism was progressive.
The nation replaced the monarch as a sovereign. In Germany and many other countries nationalism widened the "tribe" from your local fiefdom to the country.
Europe is half on its way to replace the nation with the union, widening the tribe even more. But nationalism is still too strong and holding us back.
Nationalism was also, in part, offshoot of the Romantic tradition which was a response to the 'rationalised' industrialised society, utilitarianism and other post-Enlightenment trends that constructed 'modernity'.
With any trend comes an eventual reaction and rejection as it fails to be panacea to cure all ills.
Today, European integration is not being held back by the rise of nationalism in Europe, it is what is causing the rise of nationalism in Europe.
A nondescript 'European' identity with no real defining features, historical grounding or cultural touchstones cannot survive any real crisis. Something that survives only as long as it produces clear material benefits, is always on borrowed time.
"Widening the tribe" relies on increasing levels of abstraction that weakens the ties that bind. Scale destroys all eventually.