Even in WWII the major surface combatants took years to build anyway. Look at the timelines for production of battleships and carriers. They were the complex projects of the time and they still took a long time to build despite being simple by modern standards.
Japanese ship building
From:
[Declassified] Confidential 4 June 1945 Japanese Naval Shipbuilding
www.history.navy.mil
“The building program for major warships from 1941 to 1944 shows an overwhelming emphasis on carrier construction. During this period the Jap Navy announced:
(1) Eight large carriers either built as carriers from the keel up or converted from warship hulls at an early stage.
(2) Two large carriers converted from luxury liners.
(3) One CVL converted from a submarine tender.
(4) Five CVEs converted from large passenger ships.
In addition,
Ryuho, Chitose and
Chiyoda are believed to have been converted to CVLs during this period, although no announcement of such conversion was issued.
No battleships were announced as building during this period. The
Yamato had previously been announced on 25 July, 1940. The date of
Musashi's announcement is not known.
Only one heavy cruiser, the
Ibuki, appears in this list. There has been some indication that even this ship was finished as a CVL rather than a CA.”
Germany was similar:
From:
www.globalsecurity.org
“In January 1939 Hitler approved the Z-Plan building program and subsequently abrogated the Anglo-German Naval Agreement, with the understanding that he would take all the necessary diplomatic actions to prevent war prior to 1944. The invasion of Poland on 1 September 1939, made Hitler's intentions for immediate war crystal clear and the Z-Plan was no longer a viable option. The naval building plan shifted focus to the rapid completion of the two battleships and cruiser already under construction. The submarine building program was accelerated to produce twenty to thirty U-boats per month.”
Modern ‘Total War’ would be a far more deadly proposition than WWII, but unless one side could gain a decisive victory in those few opening months, I doubt it would all over that quick.
The opening stages of WWII in the Pacific was decided by what remained of the US fleet after Pearl Harbour. The quick repair of the Yorktown and the outcome of the Battle of the Coral Sea stopped the continuous advance of the Japanese. From that point it became a ‘war of production’.
I think any future conflict would have similar outcomes. A rapid advance or series of battles until the aggressors supply lines are stretched far enough to give the defending forces an advantage. Then a slow grind from there (to either side). If you are talking about small local conflicts (invasion of Taiwan would be included in this) then yes, what exists at the start would decide the outcome. I can’t see the US/Allies staging an invasion to retake Taiwan if China held the island, so in this case future production would be irrelevant.
