In the series Deep Space 9, it has been stated that the Dominion has absolute control over their part of the Gamma quadrant. Literally this would mean that they rule every planet and race inside their borders of the Gamma quadrant via totalitarian control.
To me, it sounds like the entire Dominion is at "forced peace" aside from some examples like the Teplan. The Teplan appeared in DS-9's episode "The Quickening". The people were infected by a disease in response to their resistance to the Dominion.
But then they say most Jem'Hadar die in battle and as such it was rare for them to live past fifteen years of age. How is this possible? Your entire army controls a vast region of space which is at peace, even if that peace is forced upon it. Without much resistance, battles, and wars, how does the greater part of that army still die in battle?
Considering this was true before they entered the Alpha quadrant, they had to have other enemies in the Gamma quadrant. Can we assume they were continuously under attack by neighbouring states outside of their borders?