> a system with 2 planets with intelligent lifeforms

That is an extremely unlikely scenario because both intelligent life forms would have had to evolve before either of them developed space flight. It took homo sapiens 4 Gyr to evolve in the first place but only 100 kyr to develop space flight after that. So the odds are slim to none.

Does intelligent life mean only human-level intelligence? If we found a bunch of chimp-like animals running around, would that count as intelligent life?

Sure, but our interactions with chimp-like intelligence on another planet are unlikely to be substantially different from our interactions with chimp-like intelligence on this planet. It only gets interesting when both are more or less evenly matched, and that is extremely unlikely.

And our interactions with same-level intelligence on another planet are unlikely to be substantially different from our interactions with same-level intelligence on this planet. We've seen this before when people from one continent encountered those from another. In fiction, we show (via projection) how we might treat other intelligent life forms (every accusation is a confession).

Spaceflight is not some law of the universe. Neither are ideas of human intelligence. Whales have bigger brains, they communicate, they have culture, they have traditions. Yet we write them off and relegate them with the rest of the animals below them and make no effort towards communication, pretending its impossible as we communicate with our dogs without thinking about it as such. We wouldn’t recognize intelligent life in the universe because we set the goalposts of intelligent life to be that of a science fiction readers expectation of what intelligent life ought to look like.

Right, the biggest issue would be the unlikelihood that both planets would evolve similarly-intelligent lifeforms in the same time frames.

If Venus had a superintelligent species today, we would likely be pets or food.

If it had a superintelligent species 100,000 years ago, we will never know (or not know for quite a while).

And if it has life now that is evolving into something intelligent or superintelligent in 100,000 years, who knows if humanity will still be here to find.

I don't know why you'd think it'd be several hundred more years before communication could be done. If they can both observe each other, then all that's left is to devise a way to signal back visually. Seeing proof of one's neighbors would definitely drive people to develop ways to communicate, though I guess both planets would need to be similarly driven in order to establish communication.

Galileo was observing Venus in 1610; his telescope was enough to see the phases of Venus but probably not high resolution or quality enough to see someone waving Semaphore flags or lighting a bonfire. Astronauts in Low Earth Orbit can't see the Great Wall of China with the naked eye.

What scale of 'device a way to signal back visually' could done with 1600's era manufacturing and technology?

I'm basing what I said on the comment I was replying to which said that they _already_ figured out that they have neighbors. Given the level of scientific understanding in the 1600s, it seems highly likely that they'd have to have pretty definitive visual proof in order to know they have neighbors, which implies they can see at least some of what's going on over on the other planet. While you're right that they probably wouldn't be able to see someONE waving semaphore flags or lighting A bonfire, there must be a way to have multiple someones doing such in sync so that it'd be visible. Or more likely, a system of simple machines arranged in an array, with the ability to show either a white or black stretch of fabric. It probably wouldn't need to cover the entire area between machines.

I'll admit, it'd take a lot of effort/money to communicate and it'd be pretty slow, but it's not impossible to happen just decades (or perhaps even within a decade) of when its figured out that they had neighbors. Not hundreds of years.

One might ask how often they have a deer or a crow attempt to make contact with them. That is the expectation for other life out there, not that they’d be exactly like ourselves but perhaps more like most other life we have observed thus far.

I based my comment on the one that said "2 planets with intelligent lifeforms would've developed culturally and politically... if both civilizations grew at the same rate, 2 Galileos would've looked at the other planet and figured out "we have neighbors!"" I think that's quite a different situation than a deer or a crow attempting to make contact. It seems likely that if scientist in the 1600s were to figure out that there's life on the other planet, they're realize that the best way they could communicate would be visually. Then it would just be a matter of devising a large enough way to signal visually.

Luckily we are up gravity well from them and also have a moon as a great source of low launch cost projectiles.

Using Hohmann transfer, I believe the energy cost is the same both ways. Of course as you point out we have the Moon, "they" have none. So we'd have plenty of rocks to throw.

That said, we'd have to throw much bigger rocks to penetrate their atmosphere. And the likely (to me) actual plans would be:

Us: launch to the Moon, set up there, launch rocks from the Moon to Venus.

Venusians: launch and travel to the asteroid belt, launch an asteroid toward Earth.

The more I think about it, the more I realize that our plan would be the same as theirs: we'd both be heading for the asteroid belt, because nothing we could reasonably launch from the Moon would put a dent in Venus with that atmosphere.

And if we assume they actually can launch through that atmosphere, we're screwed: if they can do that, they're way ahead of us.

Same energy to enter the orbit, but the kinetic energy per unit mass should be a whole lot higher at Venus than Earth, so each of your shots count for a lot more. But yeah, the main advantage is the moon. And you wouldn't launch things from the moon. You would launch pieces of the moon from mass drivers.

The situation I replied to assumed they were both inhabitable planets which I assume means Earth like atmosphere on Venus. The thick atmosphere complicates things, but I don't think you actually have to hit the ground. Tunguska didn't even get near the ground and it still leveled 1000 km^2. Also if Venus has the atmosphere there's no point of a war since there's no benefit to conquest in either direction.

If the Venusians had the technology to lob rocks, they'd have the technology to do so from the asteroid belt. They don't need to mine them their own non-existent moon.

H.G. Wells has you covered here.