While it is true that Hindus have rarely been united against an external threat; it should also be noted that Muslims & Christians https://twitter.com/RanjanDubey_/status/917715069307207680

also have also not been united for most of their history. Even if you define Christiandom, narrowly, as comprising only Roman Catholicism,

it has only been united for three battles out of hundreds it has fought with Muslims.

Disunity among a big population spread over large areas was a norm, historically, while unity was rare.
The reasons for this were:

(1)Logistical problems: Logistical problems are an answer to a lot of historical questions.

When people raise questions about disunity, they are viewing a historical problem from the lenses of techno-bureaucratic state of today.

A lot of simple things, that we take for granted today, need a lot of technology & complex bureaucracy to function.

When discussion on unity in medieval times is conducted, it is invariable about political or military unity.

Both of these were hampered by logistical problems. The logistical problems of military and administration are follows:

(a)Military: Before advent of modern food preservation, armies had to sustain themselves by pillaging the land.

Grains & cured meats could be preserved, but transporting them with bullocks is so inefficient that army could be sustain itself on imported

supplies only over a small distance, or when army was too small. Harvest cycles, have historically, been a good predictor of optimum months

for invasions. Well thought out invasions were conducted when grains in enemy’s territory were nearing maturity.

All this meant that number of troops that could be fielded in a single battle were limited in number (most of the time less than 100,000)

due to carrying capacity of land being limited,and limitation of long distance supply runs.

Marathas lost third battle of Panipat,even after having a technologically superior army,because breakdown of logistics lead to starvation

of army. In his book “Art of War”, Sun Tzu recommend invading an enemy being better than fighting a defensive war due to the fact that,

while invading, you are pillaging an enemy’s territory thus harming his population;in defending,you are imposing cost in your population.

Communication technology of pre-telegram age was also rudimentary.Armies operating at a distance couldnt communicate with each other easily,

& when fighting together, could not be managed easily, especially when fighting under different commanders.

This meant that cramming lot of soldiers on battle field,when facing a competent enemy,increases probability of rout,rather than victory.

On top of it, medieval Indian armies were infantry and elephant based, and are postulated to move at a speed of 2Km to 4Km

(estimated for Mauryan Army) per day. Cavalry based armies could move ten times as fast: 20Km to 40Km in a day. Only Gujara-Pratihara and

their descendent Rajput kingdoms were cavalry based.This meant that cavalry based invading armies could force an engagements on their terms,

& withdraw easily on defeat or if conditions arent favourable (like Ghori escaped using his speed after defeat in first battle of Tarain).

All these facts mean that most of military leaders did not saw much benefit in treaties or understanding with their co-religionists

in far-away land. Even if they send help, it would take too long a time to arrive, and a large army would be difficult to sustain in field

for long. You have to depend on yourself and your neighbours alone.
Even when Hindus united against Islamic threat

-like in battle of Rajasthan in which Nagabhatta-I(Gujara-Pratihara King),Jaysimha Varman(Rashtrakuta emporer),& Bappa Rawal (King of Mewar)

united against Arab marUnmattas in 738 CE- they did not pooled their numbers into a one giant army to fight a climactic battle,

but tag teamed Arabs, with each army operating independently.

(b)Administrative reasons: It is very difficult to rule a large empire. This is the very reason why medieval history of Europe is feudal,

India was divided in small Kingdoms, and even China for most of history was feudal or divided into small kingdoms.

The technological innovations in transport and communications, which make big bureaucratic states possible, were not there.

Apart from that, by medieval times, the areal population spread was large enough that it could not be administered effectively.

One of the reason why you see large empires in Ancient era(Persian empire, Mauryan empire), but few in medieval, is because in ancient time,

most of the land was forested, and human settlement was concentrated in few places, thus the effective area that you had to administer to

control whole empire, was much smaller than you have to govern in medieval era.

On top of it,lines of communication in India were longer than they were in Europe, thus making administration even more difficult.

Mughals had full blown Jat rebellion just outside their capital, during time of Aurangzeb; and Maratha Empire had started to fragment into

feudal segments by the time of Anglo-Maratha war.

(2)Lack of a central authority in Hinduism. Lack of a central authority (like Pope was for Catholics) was a big problem

at political unification of Hindus. Even when Hindu identity had become crystallized by Mughal times, there was no central authority

that could convert that identity and sense of unease over Muslim rule into a united military and political movement.

Rajputs were sympathetic to Shivaji, but did not know what they could do for him without endangering themselves.

Look at this from perspective of game theory. In absence of a central authority, various actors could not predict what other is going to do.

In this case dominant strategy of every actor is to side to Mughals. Muslim rule is Nash equilibrium in absence of central authority.

(3)Lack of a sharply defined Hindu identity before Islamic invasion. A sharply defined Hindu identity is a product of Islamic invasions.

While concepts of Meleccha, Aryan, Dharmic were there, along with Varna system; a sharp identity, which induce hatred for and othering of

Yavan-melecchas and marUnmattas was not there. Hatred is a very strong shield, and lack of hatred could makes you open to invasion.

Now that the reason why Hindus were not united has been stated, it is easier to state what unite them.

(1)First is Hindu identity. A pan-Indian Hindu identity, is a very big uniting factor for Hindus.

The formation of this identity was due to Islamic invasion,& attack on Hindus by Christians,Commies,& Muslims is strengthening that identity

Identities are forged in fires of adversity, and every time Hinduism and Hindu gods are bashed, Hindu identity strengthen by a little.

This Hindu identity is the reason why all secession movements have happened/are happening in region where Hindus were/are in minority.

Even Tamil Nadu’s lemurism did not flared into full blown secessionism,& was pacified by small concession on language issue.

Even today, the areas which are under threat from missionaries are those where Hindu identity is weak due to high prevalence of animism.

This also extends to a sense of glorious ancient civ, which was one of the best in the world,& its destruction and degradation by marUnmatta

(2)Second: The secondary/linguistic identity of Hindus is weak, demographically & historically.

Here “strength in diversity” factor comes into play. India/Hindus have 234 identifiable mother tongues, of which 29 have more than a million

speakers. Every language, except Hindi, is spoken by a very small portion of Hindus, which makes sub-nationalism non-viable due to small

areal spread of their language, its internal divisions, & also make sure that serious attempt at secession, would get your arse beat.

It should also be noted that linguistic sub-nationalism in India is very recent, and historically, linguistic identities were weaker

(Linguistic nationalism was non-existent) than Hindu identity.
(3)Threat of external aggression. Hindus are under threat of aggression

and physical annihilation from Ummah suprastate to our west, & even non-religious Hindus are perceptive of it. This keeps people united.

(4)Passiveness. I regularly talk about how cowardice has become integral part of Hinduism in modern times, due to incessant propaganda due

to preaching of gospel of tolerance, & how revival of Kshatriya dharma is need of the hour for revitalization of civilization.

That is true, but one benefit of passiveness & cowardice is that it makes people meek. Meek people do not rock the boat too hard.

A lot of civilization degrading things like cowardice, hedonism, materialism, that rob civilization of vitality and make it weak to outside

threats, also lower threat of revolutions.Cowards & hedonists does not become good revolutionaries.

(5)Conservatism. Hindu society is quite conservative (though less than Muslim) which means that the institutions that keep religion vibrant

are in decent shape, and there are steep penalties for breaking ranks. By converting, you become a literal outcast. You would be inferior in

your new religion, & people from your old religion would despise you.

These are some points that I could think of.
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