in what common basis the national flags where invented?

Many flags look like others, and are hard to distinguish one from another. It's kind of like football uniforms, you need some kind of alternative identifier when those two teams fight.

War does not come from differing flags though. The most bloody wars are civil wars.

War is not caused by flags, it existed long before flags did...

Perhaps we should all dress alike, speak the same language, and be of one race and religion as well? Oh wait, someone already had that idea, it didn't work out too well...

All the flags of the world were made according to the beliefs and qualitys of the country. They are all different as each country has their own identity. We would not be able to make a common flag and live peacefully under it because of so many of our seperate beliefs, politics and social statuses in all the countries. For example do you think if China, Iraq and US they would live peacefully or would we just have one dominent nation take over another and use their own flag.

The ideology of flag dated to history. It means a signal to the approachers (opponents, friends, publics). Later it has developed in to the pride of the group and at last to the nations.

every nations has their own culture and the flag symbolises that. So, it is not the reason for violence. It is necessary on the other hand.

Flags have been the general expression of desire of freedom of a people of a nation, hence they are different. However, UN flag is common for its members along with their respective flags. A common flag is possible only when all nationalities merge into one international and global entity, merging their hopes, aspirations, desires into one beyond their national boundaries.

Yes i agree, time has come for the world to start thinking of a " universal flag "

If we ever had to meet aliens from another planet that were hostile, then humans might come together as a common "nation." Until that happens, petty bickering will keep tearing us apart.

Some thing got invented first and the meaning follows then.

The culture;tradition and desire of the people of the nation.

They were originally designed to distinguish armies on the battlefield