I call into question just how available 'dreadnauts' might actually be, and whether or not War1 actually constitute the majority of the armies of the 'major powers' capable of fielding 'dreadnauts'.
At just about anything below the level of an iron golem, fantasy armies can face high DR opponents. And considering the requirements for fashioning an Iron Golem, and that a beserk Iron Golem is as much a threat to its own army (and probably more) as it is to the enemy, just how common are these going to be anyway?
My assumption is that average professional armies are made of up 2nd-4th level fighters.
The first assumption is that warriors represent members of societies that have an emphasis on combat, but in which people cannot engage in martial affairs as an exclusive profession. Fighters are the full time professionals. The local gaurd or the rural tribe member might be warriors, but the world's equivalent of the Roman Legions, Spartan Hoplite, etc. are Fighters.
The second assumption is that while combat experience is the fastest way to learn the trade, training represents a slower and less dangerous way to gain experience. If training can provide 'only' 1 experience point per day to a low level fighter, then in a little over three years - even without combat - our 24 year old military professional achieves 2nd level. By the time that he retires in his early thirties, said mercenary achieves 3rd level. Elite units - and those that have experience and survived combat - can reasonably be expected to achieve 4th level. Grizzled lifetime soldiers - generally officers - might obtain an additional level or two, but eventually old age will catch up with thier strength and constitutions.
Nonetheless, large groups (in the hundreds) of 3rd - 4th level fighters even built with 15-20 PB, properly led, equipped, and supported, are a serious threat to most potential 'juggernauts'. The reason is that countermeasures for most potential juggernaut's are likely far cheaper than acquiring and maintaining those juggernauts. Assuming it can be built, an Iron Golem costs 80,000 g.p., but 1000 Adamantium arrows cost 60,000 g.p. Swarms would be very hard to control and maintain. One thousand flasks of Alchemist fire (to say nothing of a wand of fireballs) probably cost an army less than fielding swarm units, and are generally more versital. A classic 'Knight', say 4th level fighter with warhorse, full plate, lance and shield is an expensive weapon, but it is pretty darn effective at alot of things. Spirited charges easily do 3d8+9 damage on a hit, which will overcome most DR's. If I'm getting charged by 100 4th level fighters with Spirited Charge and Ride By attack, I'm probably going to take that pretty seriously.
Anything less than a full fledged Dreadnought is going to find that Brutes (orges, hill giants, war beasts, 4th level fighters with heavy lances and spirited charge) are a serious threat to them. Trents are engines of destruction, but they can be overwhelmed by a fantasy army in the Ridley Scott mold. Brutes in turn have problems against massed troops, because their AC's typically aren't high enough to provide security.
Likewise, any fantasy army that expects to meet Wizards in battle is going to have 'special forces' detailed to handle that problem, likely to include wizards of its own.
Another way you could treat it is high level casters are like nuclear weapons...they are too destructive for civilized warfare....
Funny you should mention that. My campaign assumes that sometime in the past, there was the magical equivalent of nuclear exchanges and the Gods now take a really dim view of high level spellcasters engaging in warfare. In fact, there are still large swaths of the world were wizards are burned at the stake whenever they are found out.
Actually, I think Faerun has something along those lines as well, though in practice the gods of Faerun don't seem to keep a very tight control on things because there are 30th level spellcasters blowing up cities every third Sunday in the FR apparantly.