It really depends on the level of power of the Spacemarines.
Are they going to laugh off anti-tank weapons? Are they going to die to mere machineguns? Are they beyond human limits in terms of speed?
We have a pretty accurate idea of what a Space Marine can and cannot do. They are not going to die to small arms fire, given the thickness of their armor and the sort of durability they have. They cannot survive anti-tank weapons. They die to bolter rounds, which probably wouldn't do much to an Abrams Tank without concentrated fire from at least one or more bolter rifles. A javelin should be more than enough to kill a Space Marine.
As far as speed, Space Marines are either portrayed as slow, monolithic in speed (such as the short movie they did with those Ultra Marines) or they're portrayed as about just as fast or faster. Mind you, that is in full armor. Out of armor, they are always portrayed as faster. Much faster. In any case, reaction speed and bulk are different things. Space Marines are pretty large targets on the battlefield. Expect javelins to work pretty damn well.
Flamethrowers will also work, simply because the napalm will get through the cracks of most Space Marine armor. I expect terminator armor is more difficult and hence more resistant, but I think you need something on the level of a dreadnought before napalm simply won't work at all.
Note, there are Tech Marines. There's also going to be a number of techpriests in ship engineering.
Yes, and they're all generally conservatives. In fact, they're all rabid conservatives. They're not going to immediately just start employing new technologies and strategies. There's going to be prayer, discussion, prayer, and probably a minor purge or two before they agree on how to move forward. And then some more prayer.
I'm not arguing that the tech priests are going to be useless. Indeed, if there is any existing tech that they know of which might increase the odds of winning, they'll quickly employ it. And of course, they'll employ new technology more quickly than the Space Marines. Of course, that also means the Space Marines are going to be even slower in adopting these ideas.
And again, not really big into 40k, but I don't think the Blood Angels are the sort of chapter that like to experiment with new stuff.
And they aren't stupid. Yes, they're going to look, to work out where to hit, they're going to find out what's what. Then, they'll plan on that basis.
Yes, but they're going to do so within a 40k frame of mind; which is rather narrow and direct. They're going to be able to identify military sites, city clusters that represent economic centers, agricultural areas that feed into those centers, data centers, flow of data, and trade lanes. They're also going to understand how the local planet's geography can be advantageous and disadvantageous. For example, landing their military forces in the middle of Russia is a no-no. Or engaging in a land war with China or India. Or recognizing the strong defensive value of Iran or Brazil or Taiwan.
That does not mean they're going to immediately locate enemy capitals, effortlessly hack into foreign communication systems, and perform assassination strikes on world leaders.
The most likely scenario, assuming Dante has any brains--is that Dante will realize he does not have the numbers and logistics nor the resources to conquer an entire planet such as this. Since he cannot count on reinforcements, he must make do with what he has. What he has is a full chapter of Space Marines, about a thousand of such warriors and their support staff. He cannot conquer billions with a thousand men. It's simply impossible. Even if they were all Gray Knights, he could not accomplish such a task.
Knowing that his power is at its peak at the beginning of the war, Dante will need to pick a target that is relatively isolated. There are plenty of African targets he can choose. They're low-tech, generally with young or corrupt institutions that will crumble. And those that aren't are generally small or not so advanced as to present a major threat to his own force.
The problem is that you just don't want something remote, you want something that is remote and valuable. Africa has LOTS of valuable resources the Space Marines will need, but it also so remote that it is difficult to integrate the conquered territory. He'd also be dealing with various ethnic groups and languages, which will make it harder to bring all these groups under one banner. There is also very little leverage that these African targets can place on the outside world. Material resources yes, but that's about it.
A better target is Iran. Iran is surrounded by mountains and has many valleys. That makes it difficult for any power to get their troops in. The Space Marines can deploy from orbit via drop pods, so a portion of the difficulty in transport is already solved. Their forces are also highly mobile, allowing them to move heavier equipment in without concern.
Conquering a country like Iran is next to impossible, but the Space Marines are well equipped. All of their soldiers are veterans of countless battles, including mountain fighting. They should be able to adapt very quickly. They'll also have a greater morale than the local military due to their deep religious convictions and decades of social connection to themselves and the Blood Angels chapter itself. Especially if the Blood Angels happen to land in ethnic minority areas--ie, non-Persians. Who will be less likely to resist the Blood Angels once the territory is taken.
There are other considerations as well. Iran has a strong military, but it is horribly outdated and many of their fighters are running on old spare parts, third rate domestic spare parts, or are pretty much toast. The Blood Angels can grind their military hardware into the dirt. It won't be a complete cakewalk, the Iranians have some Russian hardware too, especially for air defense, but these can be overwhelmed and taken. Especially with Terminator deep strikes. It's well within their abilities.
During the conquest, Dante will have to convert the local population. He can do so through the strong religious ties that connect them. 40k rips a great deal on the whole 'savior' process and they might be able to present themselves as sort of angelic warriors sent by God to finally wipe the heretics off the face of the Earth. The educate class will resist that, but the vast majority of the population will take one look at a Space Marine and a second look at the decimated Iranian military and make obvious calculations about survival and piety.
And all things considered, Dante will probably be pretty quick to use religious synchronicity to bridge the gap between the Space Marines and the local Muslims.
From there, Dante will eventually overwhelm the Persian ethnic group. As the Persian elites will eventually be more concerned with keeping their positions and their skin, they'll surrender and meet Dante's terms. That's where the real synergy will work. Iranian institutions are not...well, they're not the most sophisticated, but they're strong and have a rich history. Once you plug that into the Blood Angel's hierarchy, converting Iran from a third-rate backwater power that dreams of empire to a modern great power can happen within 10-30 years. We'd probably be looking at WTO era China level growth that we've seen over the past few decades.
In addition, Dante will obtain a new weapon that will allow him to dominate the Middle East. Specifically that of Islamic extremists. Iran controls a vast network. They will be the most exposed element of his whole apparatus though; the local powers will want to keep Dante in Iran, not allow him to extend outside. Still, it's hard to remove terrorists, so I expect Iran will retain a great deal of terrorists cells. However, expect the Saudi to foster terrorism within Iran. Although I expect that Dante will meet that with the typical holy purging common in 40k.
I expect that Dante will use terrorist cells in Iraq, Saudi Arabia, Yemen, Oman, Syria, and Lebanon to keep Western powers in check. Meanwhile, I also expect that he'll aggressively push into Turkmenistan, Afghanistan, Armenia, and Azerbaijan. That will piss off the Russians to the north, but Dante will by that time realize that the Russians don't have the strength to repel them. The Chinese will not be pleased about Iraq, but they don't have the projection power to really do anything about it. Eventually, I expect that Dante will push into Mesopotamia and spread throughout the Middle East.
His greatest foe will be Turkey, but so long as he moves from south to north and east to west, it will delay Turkey's reaction. And once Dante has a working industrial base and a large population to draw upon, he'll be able to supply his army with the numbers and vehicles they need to actually conquer, not to mention the more domestic tools of state via Iran's intellectual class.
But I expect that this will probably take about 15-40 years, some of which will run concurrent with their buildup of Iran as a modern power. The drawback for this, while it will bring Dante success (at least a measure of success, according to the OP's terms) is that it will allow Western powers, who are very liberal, open minded, creative, and highly motivated--to catch up technologically. As always in war and conflict, soldiers will die, weapons will be lost, and spies are plentiful. The Western powers will close the gap--and they'll have decades to do so. Probably half a century, in fact.
That doesn't mean that Western powers can or need to overtake (or even catch) Imperium technology--they simply need to become strong enough that any assault on so many enemies will simply fail, even for an Iran converted into an Imperial colony. And once that slowdown happens, then the whole endeavor runs out of steam. Dante can't overwhelm his enemies with numbers and in the long term, the most he can hope for is to keep up or remain competitive with Western tech, because conservative groups such as the Imperium do not make large leaps and bounds in technology--liberal groups do that. Because it's a function of creativity, which Imperium cults and society discourages, because it's viewed as dangerous.
So Dante's Iran will become a great power, but it does not have the resources to conquer the planet. And the time that Dante needs to build up that sort of strength will hurt him in the long term. All he can really do is establish a strong foothold upon the planet and hope against hope that he'll receive Imperium reinforcements some day.
