war is a force that we can measure
The true hero [of war] ... is force. Force employed by man.... To define force -- it is that x that turns anybody who is subjected to it into a thing. Exercised to the limit, it turns man into a thing in the most literal sense: it makes a corpse out of him. Somebody was here, and the next minute there is nobody here at all....
Simone Weil,"The Iliad, or the Poem of Force," 1945.
Suppose war is about force, and force is that power to reduce the subject to an object. We could then measure force and also the ability to wage war; similarly, we could as historians retrospectively measure the force exerted in war. We might then fruitfully compare wars on a common, definite basis.
For example, the U.S. had a military of about 12.1 million at peak mobilization for World War II, in 1945. We could consider this number as a crude measure of the ability to apply force. If we consider it that, then we see that the U.S. had slightly less military force at its disposal than the Soviet Union (12.5 million) and about twice as much as the next-largest military, which was Japan (6.1 million).
This measure of force on the field tells us a lot about who we might expect to win a war. It doesn't tell us much about how hard it is for a given society to field that force. But we can, however crudely, measure that too.
The Soviet military in 1945 represented 7% of the population; the U.S. military represented 9% of the population. It represented a larger proportionate sacrifice for the Americans to put that many men in the field. (The Japanese military, half the size of the American, represented about 8% of the population; the British, 10%.)
On this basis, we can compare the intensity of war mobilization across wars, too. The U.S. has never mobilized as high a share of its population for war as it did in 1945, nor even close. In 1865 the U.S. had about 3% of its population in uniform, and 1918 was about the same. Mobilization for Korea in 1952 was about 2.3% of Americans in uniform. For Vietnam, in 1968 when the U.S. had 3.6 million military personnel, it represented about 1.8% of the population.
For most of the peacetime cold war, the military represented between about 1 and 1.5% of the population.
The U.S. fought the Spanish-American War and the Mexican War with militaries amounting to about .3% of the population.
If at present the U.S. military is about 1.5 million strong (which makes it the second-largest in the world, after only China) it represents about .5% of the population.
In terms of relative mobilization, then, the demands of the present war look much more like those of the Spanish-American or Mexican Wars than like World War II.
As I said at the outset, this exercise provides you only with a basis for comparison. It does not tell you what conclusions to draw. I can imagine an argument that runs, we are not devoting sufficient resources to the present war, given the stakes. But that is probably not the only argument one could make.
At any rate it is probably a useful exercise to try thinking in fairly stringent terms when constructing historical analogies and drawing lessons. Anyone wishing to engage in such an exercise might consult the Correlates of War databases, as I did for this post.
Further note: Numbers of men mobilized is only, as I say, a crude measure. One might also look at the cost per soldier, or percent of GDP spent. It might be possible to calculate relative cost of war as percentage of GDP if we could be sure what the cost of this war is, but it is notoriously difficult to pin down the amount being spent at the moment. Moreover the cost of wars in economic terms have long-term ramifications. The well-known exercise by Claudia Goldin and Frank D. Lewis on"The Economic Cost of the American Civil War" is a good example of how difficult this can be to calculate. (JSTOR link)
The quotation from Weil comes from the translation by Mary McCarthy as printed in War and the Iliad by Simone Weil and Rachel Bespaloff (New York Review Books, 2005), 3.