Greetings and Peace be upon you.
over 1400 years ago in the deserts of Arab an undefeated commander of the Muslim army named Khalid Ibn Waleed رضی الله تعالی عنه was in charge of leading the Mujahedeen (those who fight for justice) into battles.
In 50 years of his life this figure titled "The Sword Of Allah" fought hundreds of battles, body full of scars, and never lost.

Source

Explain why you want to witness that historical event.

According to historians it is said that the Muslims were severely injured, only 60-30 Mujahedeen had too face 60,000 enemies.
It is said a Mujahed who was also once a enemy of Islam had a arrow in his eye but still fought. Khalid Bin Waleed رضی الله تعالی عنه broke 9 swords and killed 5000 opponents while his own sword did not even crack.
Who wouldn't want witness such bravery?

How did that event affect the world or society today?

Many People don't realize but this gave a boost to the spreading of Islam. If the Muslims had lost It would've not only wiped out the Army of strong Mujahedeen but Islam would've been considered a minority nowadays, it is Khalid رضی الله تعالی عنه's bravery that led to his soldier's survival.

If you could be there, what would you do or say?

If I was there, I won't be able to do anything. The oxygen purity back then was higher so the people had longer ages and height, I would've probably died 😅. The warriors at that time were so strong that they could not be martyred in the battlefield and were assassinated while they prayed or worshiped. Even the swords at that time were over a 100kg, it would be a big deal if I was able to lift them.
Khalid Bin Waleed passed away in 642 (born in 592 AD) at the age of 50 as a Ghazi (Ghazi is someone who is not martyred in battle but returns alive).