Border skirmishes happen surprisingly often in modern world, without leading to a war. For example, I heard there are some between India and China. And recently, Azerbaijan managed to take the whole of Nagornyi Karabah (Artzakh) from Armenia without having a proper war.
Countries are weird like that.
O . o
o . O
Yeah, I just don't see the comparison. This is more like the current war between Russia and Ukraine. Whereas Russia was pissed about their oil rights, the event in the novel was a attempt to raid the capital of an allied nation to decapitate the leadership and assassinate the Saintess who is the embodiment of the Demon Lord's seal.
The only reason it didn't devolve into fighting in the streets was because the MC destroyed the bridge. Which prompted a second raid by demons. Then yet another group when the demons failed.
I say again, the only reason the author doesn't allow the nation to move to a war footing is because then it would be much harder to justify sending the MC off on what ammounts to a suicide mission.
To achieve this effect, he fucked the king in the head and made him utterly incompetent and unable to make rational, logical or even practical decisions. This has gone beyond manga logic. The author had to twist the plot so hard that not even three obvious attempts to capture or destroy the demon Lord's seal is enough to warrant mobilizing their defenses and are just forcing the city guard to literally fight armies while the knights just sit on their thumbs in the central keep.