Hi - I'm Roman and invaded the Americans - my tech's are infantery, tanks, grenadiers, the American's is still Longbowman etc. so I am far advanced.
However I marched into their cities and nothing happened!
Their units are still there in their cities, my tanks drive into them but nothing happens, no attack.
Am I doing something wrong here?

Open border agreements are usually cancelled at the beginning of hostilities, so I really don't understand it. Is it possible that the Americans don't "see" my tanks and troups in their cities because they are far advanced?

Thanks for help...

Talk to them and say, "Your head would look good on a pike (declare war)". But note that your units will be thrown out of their borders when the declaration of war is made, so you'll have to go back in.

if you have open borders entering they cities does not cause hostilities, so open borders do not get canceled.

Its possible you can't decalre war right now, if you have some treaty - like you took tribute from them. you must wait for the treaty to wear of. Usually 10 turns on normal speed

Thanks I will try tonight.
It makes sense to declare war officially, however with Isabella I didn't have to do it.

If you don't have open borders with a civ and move units inside their borders, it will ask you if you wish to declare war. But if you do this with a civ for which you have open borders, you will just walk peacefully :)

It makes sense to declare war officially, however with Isabella I didn't have to do it.

Isabella is a pain in the a**. She went from "friendly" to "war" in one turn. (She declared on me!)

In that case, you probably did not have open borders. So, entering her land requires a war declaration. Alternatively, if you want to send someone THRU someone's land, you can requst "open borders".