There are multiple explanations behind how Durga Puja came into being. While Maa Durga has been seen to be worshipped in both the Vedic and Adivasi traditions, she seems to be absent from the pantheon of deities that were worshipped before the 18th century. By the end of the 1700s, however, the sharad Durga Puja had become one of the most important festivals for the Bengali Hindus.

Source: DNA India

The myth that picked up the most traction that explained the origin of the puja was set post-East India Company’s victory over Bengal.

A war was fought in the lands of Bengal in 1757, between East India Company and the army of Nawab Siraj Ud-Daulah, which is known today as the Battle of Plassey. After being deserted by his own men, the needle of victory tipped towards the Company. An hour of fighting later, Robert Clive defeated the Nawab's remaining forces and claimed Bengal for the Company.

Source: National Army Museum

Clive was a staunch Protestant who attributed his victory to the Divine. The only church in the city had been destroyed by the Nawab. So, Clive’s Persian interpreter, Nabakrishna Deb, asked him to offer his thanks to Maa Durga instead.

A grand celebration thus ensued at Deb’s mansion- a puja to Durga, to celebrate the conquest of Bengal by the British.

Source: The Global Calcuttan

The only source of this origin tale is a painting which was allegedly commissioned by Deb himself, which throws a light of doubt over the authenticity of his claims.

Source: Dailyhunt

But why does the battle still play such a crucial role in shaping the Durga Puja that we know today?

The rich Bengali families are known to have been performing the puja in their homes even before 1757. Back then, Durga Puja was a private affair and was seen as a symbol of status in the Bengali society.

Source: Purono Kolkata

The establishment of the Company’s rule post the Battle of Plassey gave rise to the zamindari class. The zamindars owed their position of power to the British and thus supported the Company’s rule over the land.

Source: Write Opinions

In the rural parts of Bengal, the puja used to take place in the homes of the zamindars. Their homes would turn into the place for the entire village to congregate and worship together.

Durga Pujo, thus, became an event for the zamindars to show off their wealth and power.

Source: What's up Life

Sanatan Dharmotsahini Sabha, in 1910, held a puja ceremony in Baghbazar which is believed to be the true origin of the modern-day pujo.

What followed was that a few families came together to celebrate barowari puja, which literally translates to 12 families coming together in worship. The members who took part in the puja had to pool in resources, calling for the engagement of more than a single family into organising the entire affair, quite like what we see today.

With more and more families coming together to take part in the puja, the movement spread from being concentrated to a few rich families to a wider and economically diverse community.

Source: Eenadu India

This practice later spread across the city where each household made a contribution for the pujo, running along the lines of barowari puja.

By the middle of the 20th century, it became the dominant form of worship in the city- people pooling their resources and erecting magnificent pandals for the ten-day affair.

Source: Urban News Digest

So, it is safe to say that the changes ushered in by the Battle of Plassey launched Durga Pujo from being concentrated to a few rich families into the grand affair that we know today.