Indeed, time and time again we've been undermanned and underequipped when the balloon went up. It's been the same since America's founding. Much of it it should be pointed out, has its roots in Americans bone-deep mistrust of a large military.
We've felt that way because we knew the experiences of Europe all too well and didn't want that to happen here. That's why we had no officers above Major General or Commodore until the Civil War (Washington being the sole exception and he was a Lieutenant General despite being Commander-in-Chief). We hated Britain's naval dominence so much we refused to even have one Admiral of any rank until David Farragut again in the Civil War.
And so this is how we have continued, despite the lessons of WWI and WWII when we had a very hard time at first establishing a military of size and means to meet the threat. Chief Edwards is quite correct. There is no answer that will satisfy people now. It will always be too small to start with and too large when it's done. We learn nothing and just go on as always it seems.
So what's my answer? Well take a deep breath because I may shock you, but I really do think that we could pull this off. We don't have the need to fear a military despot as we did in our nation's youth. There are far too many checks and balances for that to ever happen now. Therefore I say we should take a page from the Germans and establish a kind of
landswehr as they did of old. For those not familiar with the term, the
landswehr was a series of reserve levies of men. The Germans had three levies. Men 16 to 30 were in the first levy and at age 16 went off to serve 2 years if infantry or 3 years if cavalry in the regular army. After that they were in the first levy of the landswehr if they didn't choose to stay on in the regulars. This first levy drilled often and went on manouvers for a month or so every year much as our National Guard does today. After age 30 and until age 45 they served in the second
landsweher and drilled less and manouvered less as well. From age 45 until age 60 they were in the third
landswehr and of course drilled even less etc. What this meant was that Germany had a ready to go large army it didn't have to constantly maintain. In WWI when Britain, France and Russia were struggling to mobilize they were stunned to find Germany already on the field with millions of men. Much of the early part of the war saw victory after victory for Germany and it was only attrition that turned the tide. The Allies, while slow to do so, could prepare more of the means for war. It was as simple as that. Even so, if Germany had stayed true to the Schlieffen Plan they could have succeeded, but they didn't. They weakened the arm that was to "brush the Channel with their sleeves" to bolster the southern arm and so they doomed themselves.
As Lord Acton said, "Those who do not learn the lessons of history are condemned to repeat them" (the proper quote). We can either learn that lesson that Germans used to great effectiveness or we can go on as we have and hope and pray that strong arms and great hearts can make up the difference in the slack we go through each time we meet a new threat as they have so many times before. We can't get away with it forever.
Please note I am not encouraging a return to a draft. I think people should still volunteer but have 2 years or so of military service be an attractive option for them after high school so they do do the time and we have them in reserve when we need them. One thought is that college costs could be reduced enough that 2 years in uniform would be worth it or for those not college bound provide somekind of bonus payment at the end of service.
Of course we are doing these things now. My thought is that it be on a larger scale. For example, those who went to the first levy of reserves would have pay incentives commensurate with their greater time requirement. Those in the last levy of reserves would attain greater tax benefits. People could, of course leave at any time after each incremental time agreed to serve just as they do now. Benefits would increase as you stayed longer just as they do now, but again, on a larger scale. Spread out over time it could be done though I doubt it ever will. It's just my pipedream solution.