He built the Akal Takht at Amritsar as the seat of authority for the Sikh nation and created the Nishan Sahib (Sikh national flag) which can be seen outside every gurdwara round the world. Guru Har Gobind trained Sikhs to be ready to defend themselves and, more importantly, to defend the weak and oppressed in society whenever necessary. He kept a small army of Sikh warriors.
Guru Har Gobind taught that a follower of God cannot stand by and do nothing when their fellow human beings are being oppressed and tyrannised. If all peaceful means to end the tyranny fail then it is right and just to resort to the sword.