A) Most countries also have laws/treaties banning invasions. But when those happen to them anyway, most countries start ignoring the bans on landmines. Which are quick/cheap/easy to produce, even with century-old tech.
B) https://en.wikipedia.org/wiki/Naval_mine Not that anybody's doing Environmental Assessments on land, to determine whether the mines might be washed downstream after a heavy rain, or moved downslope by a landslide, or ...
The problem you describe sounds very much like poorly-directed mortar/artillery fire & strategic bombing - which have been killing civilians at scale since at least WWI.
B) https://en.wikipedia.org/wiki/Naval_mine Not that anybody's doing Environmental Assessments on land, to determine whether the mines might be washed downstream after a heavy rain, or moved downslope by a landslide, or ...
The problem you describe sounds very much like poorly-directed mortar/artillery fire & strategic bombing - which have been killing civilians at scale since at least WWI.