Another strategy is to secure South America and then concentrate elsewhere. I may still go ahead and make a treaty with Yellow, though a viable treaty with Brown is unlikely. So I can leave a large number of armies in Central America and then start building up in Europe.
The problem however is that your armies in Europe would put me in direct conflict with Yellow, something I may not like. This could also destabilise my treaty.
If I end up in war with Yellow, Brown may easily decide (being expansionist) that he has found the right time to attack me. My continent will be very lucrative for him, so he will always be interested in it.
Considering the above, it seems that the risks are too high to adopt such as strategy. If you think otherwise please provide your comments.