Before the twentieth century only 10%-15% of those who died in war were civilians.
In World War 2 more than 50% of those who died were civilians.
By the end of the century over 75% of those killed in war were civilians.
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Well, at least you tried to back it up this time. Figures above are correct at the time of going to press
(Source: BBC).
The conclusion drawn is that prior to the 20th century, civilians were mostly considered an asset. You occupied the invaded country, taxed them, conscripted them in your army. Nowadays the majority of wars do not aim to 'occupy' countries the same way as in the past (or simply do not succeed in that), therefore civilians are expendable.
Another reason is the power of todays weapons. If you siege a city and lob spears inside, well, that's not gonna kill as many as a few thousand cruise missiles.