TL;DR at the bottom because this will be a little long
Some people seem to hold the belief that Space Marines (infantry division mixed with Heavy TD, sometimes with artillery) are overpowered. So I played around with some tests, and found that Space Marines are actually quite easy to counter, even when they are on the defense. Yes, that's right, there's a foolproof strategy to defeat SM divisions, even when you are attacking. But first, here are my test results.
This is our Space Marine
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A fairly average template, also not coincidentally discussed in another post. The HTD is 1943 modified with +5 gun, +3 reliability. Fully researched Superior Firepower (right side) doctrine (plus the first Grand Battleplan tech since Italy starts with that and I don't know how to get rid of it with command), infantry and artillery techs researched up to 1943.
This is what we will use to defeat the SM division:
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you will notice that this division is significantly cheaper than SM - only 1110 IC vs 1932 IC. In this test, 6 SM will fight against 6 normal infantry division, so that leaves 4932 IC left over. We will use that to build 190 CAS bombers, 1944 model with +5 bombing, +2 range. +3 reliability. France in this case researches GB right path, plus the first 5 doctrines of Battlefield Support Air doc. Both doctrine trees are considered to be non-meta. French troops have 65% planning bonus, compared to 40% entrenchment for Italians.
In addition to forces described, both Italy and France gets 400 1944 fighters. Italy will set fighters to interception, in order to intercept French dive bombers. Battle takes place on a plain (not desert) province in Tunisia. Neither side has any buffs from high command, intel, advisors, generals, etc.
Let the battle begin.
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It was very close, but French troops managed to defeat the Space marines.
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As you can see, Italy lost a fair amount of Heavy tank destroyers. Yes, I changed the names of the equipment.
Now, you might be thinking that this test is a little unfair to Space Marines, since they are getting attacked from two sides. However that is to be expected when defending a front line.
However, even though Italy lost this battle, a modification to the original template allows them to defeat the attack.
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This template is much more effective against pure infantry divisions, but it's also far more vulnerable to heavy tank divisions. The point I'm trying to make is that no division is invincible. Every division template has a weakness. For example, two 20-width heavy tank divisions with no air support were able to defeat the Infantry-HTD-artillery combo.
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TL;DR If your opponent builds a lot of heavy TD's, build CAS instead of tanks. If they build a few HTD's with a lot of artillery, tanks are still effective.