Guru Nanak Dev
Just before 1500 AD, a man called Nanak who lived in India had a vision where he saw that there was no caste, there was no Hindu or Buddhist or Muslim, no religion sacrifices or rituals, no special foods, no holy books like the Quran or the Rig Veda, but that God wanted everyone to reach out to God for themselves. Everybody was equal to everybody else: men and women, rich and poor, Sikh or Hindu or Muslim. There was only one God. (You might compare this to what Martin Luther was saying in Europe at the same time.)
Nanak soon got more and more followers. He called his followers "Sikhs" and they called him "Guru Nanak". Guru Nanak told the Sikhs (SEEKS) that they should live normal lives with wives and children, not become monks. Sikhs showed their commitment to equality by serving free meals to anyone who showed up. Like Buddhists, Sikhs believed that people were reincarnated many times, and that by being good you could escape reincarnation and become one with God.
After Guru Nanak died, there were nine more Gurus (all of them were men, even though women were supposed to be equal to men). The Sikhs began to fight with Hindu kings because the kings didn't like the idea that everyone was equal. The Mughal Emperor Jahangir didn't like this idea either, and persecuted the Sikhs, killing some gurus. Many Sikhs became fighters to resist the Mughals.
The last Guru was Gobind Singh, who died in 1708 AD. Gobind Singh formed a group of special Sikh people who had been baptised and were extra-religious. These people didn't cut their hair. Sikh men began to wear their long hair inside a special cloth turban. Sikh women weren't supposed to cut their hair or shave.
Just after 1700 AD, as the Mughal Empire got weaker and weaker, some Sikhs formed their own Sikh Empire, occupying the Punjab in north-western India. This independent Sikh Empire lasted until 1849, when the British Army conquered them in battle.
Sikh soldier in World War I
But after the British conquered them, many Sikhs fought for the British against Hindus. A large part of the British army in India in the late 1800s was made up of Sikhs. Many Sikhs volunteered for the British Army to fight in World War I and World War II.
After World War II, these same Sikh soldiers decided to leave the British army and fight for India's independence. After India won its independence, the Sikhs were upset to find that the Hindus remembered how the Sikhs had fought against them before, and treated them very badly. That fight is still going on, with Sikhs still being persecuted by Hindus in India today.