The very first limitations on interstate use of force
date back to theories of "just war",
which had existed since antiquity,
but were mainly developed in the Middle Age by the Christian church,
notably in relation to religious wars such as the crusades.
According to those theories, in so far as a war was just, it was legal.
For example, according to a Christian thinker such as Thomas Aquinas
a war was just when it was authorized by a lawful authority,
it intended to achieve a "just" outcome
and its ultimate aim was to restore peace.
It was therefore, theoretically at least, possible to distinguish between the just belligerent,
whose cause for resorting to war was just,
from the unjust belligerent, whose cause was unjust.
Once it was possible to distinguish between these two types of belligerents,
thinkers often gravitated towards the view that the "just belligerent" should be privileged
to the extent that it should not be subject to all the constraints under the jus in bello.
The "unjust belligerent" by contrast was to bear the full weight of the law.
This sort of argument is termed the asymmetrical application of the law of war.
Despite holding a certain superficial attractiveness,
the idea of an asymmetrical application of jus in bello in favor of the just belligerent
was a clear obstacle to both the application and development of jus in bello.
The first problem with an asymmetrical application of jus in bello
is that it could prove very difficult to establish
which party's cause was "just"
and would therefore enjoy the privileged status of "just belligerent".
This was particularly difficult because
many just war theories involved a very personal sense of grievance
and so were highly subjective.
Unsurprisingly, it was rare for warring states to agree as to whose cause was "just".
This is still valid today.
The second problem with an asymmetrical application of the jus in bello
is that it runs against the idea of mutuality,
reciprocal assertions of right
and reciprocal expectations of protection,
which is at the basis of respect for jus in bello.
Basically, belligerents were ready to respect jus in bello
because they expected that the enemy would do the same toward them.
Any asymmetrical application of jus in bello would expect one belligerent
to respect the full jus in bello obligations
that its enemy is free to disregard.
Given the importance of small advantages in war,
there would be little incentive
for the state with more onerous obligations to comply with them.
This in turn would create the risk of a downward spiral
in terms of compliance with the rules.
