My question is what, if any, factors/events (aside from contact with
another civilisation that uses different tactics and/or weapons) would
lead to a military force building ships that use armour and/or hull
shapes designed to defend against weapon systems that that force does
not itself use?
Events That Trigger Innovation
Small, Far Away Conflicts Involving Others
In times of relative peace, very small scale conflicts far away and accidents that illustrate graphically military vulnerabilities can have a profound impact.
The fundamentals of trench warfare tactics, for example, were invented in New Zealand in what are now called the New Zealand Wars:
The New Zealand Wars were a series of armed conflicts that took place
in New Zealand from 1845 to 1872 between the Colonial government and
allied Māori on one side and Māori and Māori-allied settlers on the
other. They were previously commonly referred to as the Land Wars or
the Māori Wars, while Māori language names for the conflicts included
Ngā pakanga o Aotearoa ("the great New Zealand wars") and Te riri
Pākehā ("the white man's anger").
This conflict happened, literally, half a world away from Europe between factions utterly unfamiliar to most Europeans, and was on a tiny scale compared to WWI that would follow, but a few cutting edge military planners studied the tactics used then and those tactics soon became dominant in WWI in short order.
Similarly, the Falklands War, even though it occurred at the opposite end of the Earth from England, didn't last all that long and resulted in only 1000 deaths, did more to influence warship design and naval strategy, and the logistics of power projection, than any other post-WWII conflict. Most importantly, it illustrated the immense advantage that a single skilled submarine outfitted for war could have over a substantial fleet of surface ships.
At even smaller scale, civilian troublemakers who have targeted commercial aircraft with high powered laser pointers that could be used to blind pilots eyes have led to countermeasures against this threat in military aircraft even though this kind of attack on pilot sight has never actually been used in warfare.
Accidents And Natural Threats That Present Obvious Military Lessons
But, not all of the events that lead to radical innovation are actual conflicts. The Hindenburg disaster brought the use of hydrogen inflated airships in war to an end before it had even begun (even though the flammable material used for the sheath rather than the inflating gas was the main issue in reality).
The Titanic's collision with an iceberg led to a radical rethinking of the importance of adequate lifeboats and evacuation procedures for sinking ships that influenced civilian and naval ships alike and also commercial and military aircraft design precautions for water landings that followed.
Accidental collisions of U.S. Naval ships with civilian ships in peacetime have led to major changes in how crews are prepared for readiness.
Civilian commercial aircraft accidents have led to numerous innovations in how military aircraft operations are run and how military aircraft are designed.
In a space setting, deadly accidents, for example, from impacts with space debris or inattention of safety issues with solid rocket boosters, would alert planners to the same kind of potential risks.
As another space example, the fear that an asteroid or comet impact could caused massive harm to Earth as it has multiple times in the past could lead to the development of anti-ET object technologies with obvious military applications to new threats.
Motives That Trigger Innovation
what, if anything, would make taking measures "worth the effort"
before you meet an organised military force
Necessity is the mother of invention and efforts to anticipate new military threats usually involve a perception that there will be a near term need to use military force. But, it is also important to recognize that this motive need not e widespread. It only needs to e widely shared among political and military decision makers who have considerable personal power.
A Strong Inclination Of Leaders To Use Military Force To Achieve Political Ends
Lots of the historical examples involve the sponsorship of military innovators by political leaders who anticipate being the aggressors to conquer previously insurmountable barriers to military success, or at least, are not so much pro-military as pro-use of military force (two very different things).
For example, the Third Reich invested in military innovation that revolutionized modern warfare, because they planned on using it well in advance, and at first in WWII, it gave them an edge.
Far earlier, the Hittites did something similar, developing innovative metallurgy techniques for their weapons and coming up with chariot warfare tactics, because they planned on conquering as much of Anatolia, Mesopotamia and the Levant as they could, while their opponents only started to innovate once the fighting began.
Military innovation by non-aggressors in the WWI, and WWII didn't have top leadership support and financial support until those wars were in progress. Neither side innovated much before combat began militarily before the U.S. Civil War because most people on both sides didn't think it would happen with any vigor. Vietnam was another war that was unanticipated for U.S. forces leading to military innovation only once the war began.
Widespread Anticipation Of Isolation In Foreseeable Future Conflicts
On the defensive side, military innovation is frequently triggered by a perception of a clear and present dangers that the defending military can't count on anyone else's help to protect them from.
After Hiroshima made the threat of nuclear weapon a widely anticipated one, both the U.S. and its allies and the U.S.S.R. and its allies innovated in earnest prepping for a widely anticipated nuclear war that never was.
Some of the countries that have contended "far about their weight class" so to speak in terms of military innovation have been Israel, which has faced ongoing rhetoric from its neighbors who say they want to drive it into the sea; South Africa, which lacked international allies during apartheid and also had good reason to fear an insurgency; Iran which has spent much of its recent history diplomatically isolated, with vocal threats from the U.S. and Saudi Arabia, and Switzerland, which has built a foreign policy around self-reliance and neutrality achieved by not entering into alliances.
What did this lead to?
Israel basically invented the anti-missile missile, invented military concepts that include large numbers of women as soldiers, devised several new kinds of anti-terrorism tactics, and did more to develop the art of the air to air fighter battle than any other country since WWII.
South Africa invented what the U.S. called when it adopted them for use in Iraq and Afghanistan, the MRAP (mine resistant, ambush protected vehicle), as it prepared for counterinsurgency fights that it anticipated.
Iran pretty much invented the concept of the small, fast, coastal, heavily armed, lightly crewed missile boat, and the widespread use of non-nuclear coastal submarines to effectively threaten much more powerful military surface warships.
Switzerland has developed dozens of defensive military systems and strategies found nowhere else in the world, with one of the world's largest citizen's militias, large underground bunkers with caches of weapons and supplies and room to evacuate citizens too, bicycle based anti-tank weapons, and all manner of other response to the threat of an invasion of its country with conventional military forces.
The tangible fear of conflicts that were widely anticipated and very specific led to innovations that countries with less clarity of mission and more promises of military support from allies did not.
Small, Non-Meritocratic, Non-Bureaucratic Political Organizations
If you're not actually fighting wars, you design your weapons systems
to protect against the most dangerous stuff you can think of. This is
what the US Navy has been doing for the last 80 years.
Not really. If you're not actually fighting wars, the norm is that you design your weapons systems to win the last war that you fought, whether or not it makes sense to do so. The U.S. Navy is a prime example. Only modest steps have been taken to respond to multiple post-WWII threats, and the predominant ship designs date to the 1980s, about 35 years ago. There have been minor upgrades but not major rethinking.
There is a sociological reason why this happens. The people who are promoted to make decisions going forward are the people who were most successful in the last war, whether or not those traits are valuable going forward, because people promote folks who are successful rather than people who weren't. But, the people who were most successful in the last war are the ones who are most wedded to old technologies and tactics. For example, in WWI millions of lives were lost when generals insisted on using cavalry charge tactics long after it became clear that the machine gun had made them obsolete.
In contrast, the U.S. Army has fought many wars since WWII and has radically remade itself as a result in response to what has and has not worked in recent conflicts, for example, replacing the heavy tracked tanks designed with WWII in mind, with lighter, air transportable wheeled vehicles, which in turn were supplanted by vehicles adapted to IEDs and ambushes with armor piercing bullets. Meanwhile, anti-aircraft guns and artillery that were prominent in WWII faded as the U.S. Army always had air superiority and as cruise missiles and aircraft based smart bombs made howitzers obsolete.
Also, in the absence of actual experience (personal or merely observed in others) with asymmetric warfare, military tech procurement planners unconstrained by the hard reality of actual war often fall into the mental trap of planning for "fair fights" against similar systems.
Tanks designers dream of tank v. tank wars, even thought tanks were invented as anti-infantry weapons. War ship designers dream of blue sea surface combatant duels, even though submarines, aircraft, mines and hypersonic missiles are the real threats. War plane designers dream of dog fights, even though one shot, one kill before the enemy knows you're there has been the leading approach in actual air warfare since Vietnam.
Enlightened military leadership is rare and usually comes down to a handful of innovators who are ignored and derided until they somehow secure power.
In these situations, overcoming the natural sociological and organizational instincts to fight the last war, and to prepare for "fair fights" with weapons facing off against opponents with similar weapons, you need to have some mechanism, almost a corrupt one, that overrides conventional uninspired bureaucratic conceptions of meritocracy.
Basically, the happens when some rare military innovator has a power base or political patron who can overcome the natural inclinations of a military bureaucracy.
Outside of times of actual war, this is something more prone to happen in a monarchy where people are promoted for reasons other than perceived talent, than in a democracy or any other technocratic state. For example, a powerful crown prince in a not very large country may have more of an ability to throw is support to a maverick military innovator than a senior civil servant or high ranking soldier in a large technocratically run, not particularly corrupt, highly bureaucratic military force.
It is more prone to happen when power is securely concentrated in a small number of key political leaders than when it is dispersed which requires more collective action to overcome barriers to innovation.
It is more prone to happen in countries with smaller military forces than it is with larger once, since bureaucratic inertia isn't as great and there are fewer senior official who must be won over to secure change, subject to the caveat that certain kinds of innovations are possible only with the huge military budgets only available to the largest and most economically powerful countries.
For example, military innovation in the early modern period was greater in independent micro-states with hereditary leaders in what became Germany and Northern Italy prior to the late 1800s, than it was in countries that had been large for a long time like France and England.
Innovation In Other Fields
Another way that military innovation against previous unanticipated threats can develop is as a side effect of health non-military innovation.
For example, a large civilian economy driven revolution in computers and electronic communications from about 1960 to the present, has given rise to technologies that have been easy to adapt to military ends once developed. These technologies also often trivially imply the need for new kinds of defenses (like cyber warfare defenses that evolved to a great extent out of criminal and accidental breaches of security in civilian systems).
The development of a standardized modern rifle in the 19th century (and innovations like the machine gun that this made possible) was largely a product of manufacturing technologies developed in the civilian sector that made it possible to mass produce goods made of interchangeable parts.
Going back much further, the naval superiority of the British, Spanish and Dutch fleets largely derived from innovations in ship building technologies developed initially for commercial applications.
A generally prosperous economy driven by civilian innovation also makes it much easier to raise tax funds that make it easier for politicians to budget for guns instead of butter, decisions which can free up the funds needed to finance expensive military innovations.