Fuchs:

And why exactly should it be more difficult to maintain peace when it's harder to mobilize the population for war? The argument seems to go into the entirely opposite direction in my opinion.
It's more difficult to maintain peace because the weaker nation state cannot deal with and neutralise it's own internal rivalries that may spill over it's own borders. These rivalries arise from differences in ethnicity, region, political or religious beliefs.

Examples of internal groups include:

1. Hezbollah - which has both political and military power - and started their own war with Israel against the wishes of the Lebanes Government which was powerless to stop it.

2. The Muslim Brotherhood in Egypt. This is effectively a social movement against official corruption.

3. The entire range of Afghan warlords.

4. The Kurds.

5. And of course the Taliban.

The list is pretty much endless, but all of these groups have the ability to seriously destabilise their own state as well as make trouble for others - compromising their Nations ability to make, and keep, peace with their neighbours.

...And as others have pointed out, the most obvious way of attacking any rival is to look for such groups and support them, as we have, sadly, all done before.