Let's take a basic model; two countries coexist.
It's impossible for both to be vastly superior in military power to pursue their own security. Only one can be vastly superior at a time, and the attempt to reach this superiority may lead to an economic lose-lose situation. Even the case that one nation gains such an upper hand is messy; warmongers would push for an abuse of this power.
The most stable case would be if these countries developed armed services which excel much more in defence than in offence, so both countries would be safe if they have approximately equal warmaking potential. This is very difficult to achieve with modern military technology.
Reality is even more destabilising; aggressors have intrinsic advantages ceteris paribus.
Here's an incomplete list, which I expect to link to in later posts:
- timing (procurement): The aggressor can choose the timing of the procurement of new technologies in order to reach a technological superiority during a certain time window
- timing (season): An aggressor could train and equip especially for one season (such as wintertime), gaining a specialisation advantage during this season.
- timing (day): The aggressor can choose the first day of conflict to his advantage, example Yom Kippur War.
- concentration before counterconcentration: The aggressor can amass his forces in the border region and the defender can react to this only with a lag (detection, communication, decisionmaking, distribution of orders)
- choice of theatre: The aggressor can choose where to attack, and whether to draw small powers into the conflict. He may limit the conflict to a single region if it suits his strengths, or escalate widely if he thinks this would suit his strengths. The non-aggressive defender would not forcibly draw additional neutral powers into the conflict.
- unusually high readiness at day of aggression is possible: Aircraft, ships and heavy land forces equipment often have readiness rates of 50-80% in peacetime, but this can be pushed to 80-95% at a specific date, for example for maximum power available on the day of aggression. One example was Germany in 1940; the readiness rated of the Luftwaffe were artificially high in the days prior to the campaign in the West.
- strategic surprise: This goes well past mere timing; an aggressor can desensitise the target to the telltale signs of attack preparations by repeatedly showing these without attacking for real. This, too, happened in 1940 - the multiple delays of the German campaign in the West desensitised the French to the (often correct, but soon obsolete) intelligence reports of German attack intentions.
- aggresssor gains bargaining chips: The aggressor will likely make some territorial gains, which in the event of a truce will serve as bargaining chips or may even be occupied indefinitely