Why the EU will never be united. Perhaps history could give an indication.

Maulana Abul Kalam Azad

And they say history does not repeat itself. The autobiographical narrative 'India Wins Freedom' of Maulana Abul Kalam Azad, then leader of the Indian National Congress during the Indian independence movement and following India's independence, the first Minister of Education in the Indian government, might prove this wrong.

On page 227 of his autobiography Maulana Azad cautions that even a strong religious identity (read today Idea of a United Europe) fails to unite a nation (let alone a group of nation states) when certain conditions are not conducive. Below is a quote of his book and the similarities with the situation in Europe are conspicuous.

Mr Jinnah and his  followers did not seem to realize that geography was against them. Muslims in  undivided India were distributed in a way which made it impossible to form a separate state in a consolidated area. The Muslim majority areas were on the north-west and the north-east. These  two regions have no point of physical contact. People in these two areas are completely different from one another in every respect, except only in religion. It is one of the greatest frauds on the people to suggest that religious affinity can unite areas, which are geographically, economically, linguistically and culturally different. It is true that Islam sought to establish a society which transcends racial, linguistic, economic and political frontiers. History has however proved that after the first few decades, or at most after the first century, Islam was not able to unite all the Muslim countries, into one Slate on the basis of Islam alone.

He even offers advise to the people in today's EU on page 238:

QUIT  INDIA the EU
The All-India Congress Committee government of country X has given the most careful consideration to the reference made  to it by the Working Committee referendum in their resolution dated July 14, 1942 any time in 2017 or 2018, and to subsequent events, including the development of the war situation chances, the utterances of responsible spokesmen of the British Government EU, and the comments and criticisms made in India our country and abroad. The committee government approves of and endorses that resolution and is of opinion that events subsequent of it have given it  further justification, and have made it clear that the immediate ending of British EU rule in India our sovereign country is an urgent necessity, both for the sake of India our country and for the success of the cause of the United Nations peace and prosperity.  The continuation of that rule is degrading and enfeebling India our country and making her progressively less  capable of defending herself and of contributing to the cause of world freedom.

INDIA WINS FREEDOM
An Autobiographical Narrative
MAULANA ABUL KALAM AZAD

Orient Longmans Private Ltd., 1959