The Union Flag of the United Kingdom...
Well, I could spend a great deal of space talking about this one!
It is essentially made up of three flags:
England&Wales (the red cross on white background you'll have seen at Euro 2004) or St George's Cross
Scotland - blue background with white diagonal cross also called the Saltire
Northern Ireland - the red diagonal cross on white background
It is said that the name "Union Jack" comes from the time when it was used (mainly) at sea and was hoisted upon the jack staff. Others say it comes from King James (Jacob), and yet others have also associated these isles with being the place where the Union of the tribes of Israel (i.e. Jacob) have gathered.
The colours all have symbolism to them, and the religious traditions associating it with Israel (the people of Israel) are quite persistent. See, for example:
https://www.1335.com/unionjack.html
This subject would need to be looked at with heraldry and British history and traditions too.
Ah, but that's enough to whet your appetite.
Dubhdara.
I agree with LDS Forever with the history of Argentina flag. However I was taught that the light blue & white stood for the sky & the clouds. Also I had heard that the white stood for the mountaintops with snow (I would imagine they're talking about the Andes mountains or Bariloche).