A common argument in favour of spending billions on a navy is that we depend on maritime trade for sustaining our wealth.
Well, actually much more of German exports and imports happen by air cargo than maritime shipping (measured in value, not mass, of course). Much of our trade by land came from or ends up at foreign ports such as Rotterdam, though.
It's also noteworthy that our trades would take a hit in the event of war regardless of whether and how well maritime trade was protected. Even the financial markets cramps of 2008/2009 greatly reduced trade globally.
But that's mere background noise to the central point I want to make:
Maritime trade simply doesn't work the way it did up to the Second World War.
Back in 1930 a German-flagged ship would take in crates in an Asian port and the steam towards Hamburg with but a few stops to take new coals onboard (there were diesel-driven ships as well, of course). A naval blockade and a high seas submarine war against German trade were easily possible, even thousands of nautical miles away from Hamburg.
Today, a Panamanian-flagged container freighter would be loaded with containers in some Asian port, maybe have some unloaded and others taken in at some Arabian port, pass the Suez Canal, have containers unloaded and new ones loaded in some Mediterranean port, cruise to Le Havre, have containers unloaded and loaded at Le Havre, cruise to Rotterdam to have many containers unloaded and loaded and then keep going on.
Equivalently, in 1930 a German-flagged tanker may have depart from some Black Sea port after loading Caspian Sea crude oil, would cruise to Hamburg and unload. There was no really global oil trade. Nationally-anchored oil companies and even trade deals between governments ensured that but a small fraction of the global supply was really traded on markets. Much of the oil trade was rigged, locked up.
Nowadays a Barbados-flagged tanker might load crude oil in the Persian Gulf and set to cruise to Rotterdam, but while en route some new trade deal between brokers was finalised and the captain gets orders to change course to Marseille instead, and unloads 200,000 tons of crude oil there. There are traditional trade links in the oil business, but a huge share of the global oil trade is traded on a global crude oil commodity market.
The 'simple' trade schemes of 1930 are also representative of 1913, and they were still intricate and difficult enough that naval warfare in a mostly European war became the final straw (and excuse) for the United States to foolishly and pointlessly join the first World War.
The global trade logistics of today are incredibly much more complicated. It would be fairly possible to create a close blockade within maybe 1,000, or at most 1,500 nautical miles distance from European harbours, but even that would greatly infuriate many neutral nations.
To attempt a naval trade war against Europe in the Indian Ocean would guarantee that just about every major power world-wide that's still neutral would be infuriated. Even your allies may be infuriated by hit on their maritime trade.
A container ship in the Indian Ocean may have loaded 80% containers meant for Europe, but you cannot really frisk it, or it turns pointless once government-supported efforts conceal the origin and destination (even content) of containers. To sink that container ship would still sink 20% "wrong" containers, with exporter and importer infuriated.
It's similar with crude oil trade; you might intercept a ship that's meant for Europe, but unless you take it over as a prize that oil will be lost. That's like a reduction in global crude oil 'production', and would increase the prices because a global market is like a system of connected pipes. Still, a 'Barbadian' tanker captured in the Indian Ocean might officially be destined for Odessa, Ukraine and be re-routed to Rotterdam once in safe Mediterranean waters.
So how exactly would for example China wage global naval warfare against Europe? And if not China, who else? India? Same problems. Russia? All its European ports would be taken out b air strikes. Warships -submerged or not- would hardly be able to embark on a second war patrol, and the damage done on their first one would be tolerable.
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My stance is that we need no warships to secure trade in the Red Sea. That's the job of the regional powers; Egypt and Saudi-Arabia. We don't need warships to secure trade in the Strait of Malacca. That's the job of the regional powers; Indonesia and Malaysia.
Frankly, a mere four ASW frigates and four AAW frigates aren't going to make a dent in any naval warfare plans for any at least semi-reasonable scenario.
Even the ridiculously oversized U.S.Navy may boast that it protects world-wide trade, but if actually challenged it simply couldn't. Almost all its combat ships are needed to protect its supercarriers anyway. There's a reason why thousands of escorts were created in both World Wars; peacetime navies simply do not include enough escorts to protect oceanic trade.