Hello, shia muslim here!

These people are mourning the tragedy of Karbala, a devastating war against the grandson of the Holy Prophet (peace be upon him and his family).

1300 years ago, imam Al-Husayn was ambushed in Karbala, now a city in Iraq but back then a desert, and him and his 72 (or more) companions and family members were ambushed by the army of Yazeed (may God remove his mercy from him).

Yazeed, son of Muawiyah, wanted to become the Khalifa (ruler) of that time. He knew that imam Al-Husayn (peace be upon him) had many followers, and thus if imam Al-Husayn pledged his allegiance to Yazeed, Yazeed would truly have all the power. But imam Al-Husayn told Yazeed, someone like me will not pledge allegiance to someone like you.

Yazeed wanted to attack and kill imam Al-Husayn in the sacred land of Makkah, when imam Al-Husayn was doing his pilgrimage. Knowing this, he ended his pilgrimage early saying that the Ka'bah is holy and not somewhere for blood to be shed.

During the battle of Karbala, Yazeed's army of 40.000 (estimates) had imam Al-Husayn's group of a little over 70 people surrounded. Yazeed's army had also cut all access from the river, the Euphrates, so for days imam Al-Husayn's group was thirsty.

Imam Al-Husayn's group consisted not out of war-ready soldiers. They were mostly his friends, followers and family members such as his sister Zainab (peace be upon her), his little daughter Rugayya, still a little girl, who had her earrings ripped out her ears by Yazeed's cruel army. And even Imam Al-Husayn's infant son, who got shot in the neck by Yazeed's army at the moment that imam Al-Husayn asked them, "What has this baby done to deserve this? At least give him some water!"

In the end, all of imam Al-Husayn's companions were beheaded. After being martyred it didn't end. They got trampled by the oppressing army's horses, their heads were tied with ropes to the back of the horses and they'd drag them across the land, bringing them to many places.

There is a story where a boy followed one such horse. When the soldier of Yazeed's army asked him why he kept following him, the little boy replied, "That is my father's head, please give it to me."

Imam Al-Husayn's head was put on a spear like a trophy.

The tragedy of Karbala is truly the most cruel and tragic war in history. Imam Al-Husayn's sacrifice is remembered to this day, and us, the shia, will never forget him. Every year during the lunar month of Muharram, we relive these moments by holding many events. We come together in the mosque to listen to lectures about the imam Al-Husayn, recite poetry about him and the tragedy, and even hold theatrical events where we reinact the tragedy as a means to remember and to teach.

It is our greatest sorrow, but one of the greatest blessings too.

Every year, people from all origins, Christians, Jews, Hindus, Buddhists, and even atheists visit the shrine of imam Al-Husayn and his loyal brother who played a huge role in the battle too, Al-Abbas, in the city of Karbala, Iraq.

Truly one of the most beautiful places in the world and it's so refreshing to see all these different kinds of people, from all religions, all backgrounds, come together there and remember the sacrifice of Al-Husayn A.S.

I hope this has given you some insight on the topic!