I have not really played any diplomacy games so far,
but based on my experience in other computer games and the chain of messages that you have described,
**Apparently, the owner of USA thinks** that an alliance means that USA and Europe will work together on the battlefield, and do things like attack Russia together... hahaha :D
or that Europe will be asked to help USA by attacking Africa to stop the owner of Africa from getting South America, (and threatening USA)... either by attacking the owner directly, or even merely blocking the path of Africa's owner.
When the owner of USA suggested a peace treaty,
**that implies that the owner of USA thinks** that the USA and Europe will avoid attacking each other's territory, and perhaps nothing else.
However, in order to avoid easily breaking the agreement,
a peace treaty often extends to negotiating and agreeing on claims:
Such as: You can have Africa, but then I want China and South America...
and how we achieve those goals despite the other players in the game is up to each of us...
In most computer games that I know of, an alliance is the stronger agreement, and a peace treaty is the looser agreement, as I described above.
Now, what I or others in this thread think about this topic is a very useful discussion to have for you and for us,
but what's most important for this example is what the owner of USA thinks a peace treaty is supposed to mean, and what an alliance is supposed to mean... and what it means to them. :)
I hope this helps.
EDIT: Hahaha, these other two have it in a nutshell. :)
Edited 6/25/2015 05:50:44