Has there been a war whose declared goal was a mere land grab without any attempts to ethically justify it?

Where to start - and where to end.

Let's start in 600 A.D., and consider:

• Muslim Arab conquests of Arabia, Syria, Persian Empire, Egypt, Roman/Vandal North Africa, Berber Africa, Saharan West Africa (ie Mali et al), Sicily, Iberia, and Provence.

• Hungarian conquest of the Hungarian Plain.

• Frankish conquest of France.

• Carolingian conquest of Germania and Northern Italy.

• Mongol conquest of most of Asia north of the Himalayas.

• The Spanish & Portuguese Reconquista.

• Seljuk conquest of Anatolia.

• Ottoman conquest of Balkans.

• Crusader (ie Frankish-Norman) conquests of assorted Middle-East and Balkan states.

• Moghul conquest of Northern India.

• Frederick The Great's seizure of Silesia.

• Dutch conquest of Indonesia.

• British conquest of India.

• Three partitions of Poland by Russia, Prussia, and Austria.

• American conquest of everything between the Mississippi River and the Pacific.

• Chinese conquest of Tibet.

• Russian conquest of Siberia, Kazakhstan, a half dozen other "stans", Siberia, Karelia and Finland.

Have I left anyone notable uninsulted? I hope not; as the petty excuses of militaristic adventurers are, in the long run, how "natural selection" of government occurs.

What you are looking for does not exist: even if nations can became "monstruous" as for example Nazi Germany, they always justify their acts by something else that just "I'm hungry of lands". Still, there are some examples of wars plainly justified by "I want this land" with no (or little) ethical justification nor instrumentalization.

• The Nazi invasion of USSR : The ideological background of this invasion is strong, with all the Nazi ideology behind. But the final reason for the invasion is, from the German's point of view, very basic: "Germany needs Lebensraum [minimum space to live and be powerful] so will take it from the Untermenschen [lower men, for example the Russians, and Soviets and Slavs in general]
• Argentina invasion of the Falklands: Argentina tried to justify the war as an aggression of its soldiers that were exerting their right to grab these lands

Still in these two examples, Nazi Germany and Argentina thought (or at least said they thought) that they had an ethnic or historical right to grab the land, even if it was against current international laws. A third example is slightly different:

• Iran-Iraq war: the reason and the public justification for Iran-Iraq war was quite simple: Iran is supposedly weakened by the Islamic revolution and civil war that ensued, and there is a past agreement on a contested territory. Saddam Hussein started the war with no other reason that "There is an opportunity to contest the agreement over Chatt-El-Arab [the contested land]".

Please note that after the beginning of the war, other justifications came in: Sunnis versus Shiites, protecting the Gulf from Iran's expansion of revolution... And there was still a reference to history and ethnicity of the Chatt-El-Arab populations made by diplomats of Iraqi side to justify that Iraq should control this land

Of course, this has happened before. Examples:

The Mongol Invasion of the Mamluk Sultanate began with the following letter:

From the King of Kings of the East and West, the Great Khan. To Qutuz the Mamluk, who fled to escape our swords. You should think of what happened to other countries and submit to us. You have heard how we have conquered a vast empire and have purified the earth of the disorders that tainted it. We have conquered vast areas, massacring all the people. You cannot escape from the terror of our armies. Where can you flee? What road will you use to escape us? Our horses are swift, our arrows sharp, our swords like thunderbolts, our hearts as hard as the mountains, our soldiers as numerous as the sand. Fortresses will not detain us, nor armies stop us. Your prayers to God will not avail against us. We are not moved by tears nor touched by lamentations. Only those who beg our protection will be safe. Hasten your reply before the fire of war is kindled. Resist and you will suffer the most terrible catastrophes. We will shatter your mosques and reveal the weakness of your God and then will kill your children and your old men together. At present you are the only enemy against whom we have to march.

Italy embarked on its adventures in Abyssinia (modern-day Ethiopia) because it had recently unified and was "behind time" compared to its fellow Great Power European countries in the scramble for colonies:

Once unified as a nation-state in the late 19th century, Italy intended to compete with the other European powers for the new age of European colonial expansion. It saw its interests in the Mediterranean and in the Horn of Africa, a region yet to be colonized and with access to the ocean. Italy had arrived late to the colonial race and its status as the least of the Great Powers, a position of relative weakness in international affairs, meant that it was dependent on the acquiescence of Britain, France and Germany towards its empire-building.

There are no stated objectives in Qin's wars of unification at the end of the Warring States period, as well. The only objective is unifying China, which necessarily involves "we are stronger than these upstart states so we're going to conquer them".

It has long been trendy for nation leaders to come up with some sort of ethical justification when starting a military invasion to a neighboring country.

That has always been the case. The Romans never went to war, unless it was justified, at least from the Roman point of view. The other point of view was irrelevant. The justified war concept goes back long before the Romans.

The fact that you can't find any example of what you want (a pure land grab) shows humans everywhere always feel a need to justify what are essentially criminal actions.

The problem with your question is that you only have to go back a few hundred years to find that an invasion for purposes of a "land grab" did not have an "evil, criminal character" it was simply a part of normal life. If a leader was a good leader (meaning strong leader) they were expected to acquire neighbouring territory for himself, be it by war, diplomacy or other means. And it mostly was "for himself", not for the nation - the nation state being only about 500 years old, and rulers could rule many "nations". Having a new ruler did not change the "nation" you belonged to.

Going back only a few centuries the name or nationality of the supreme ruler made very little difference to the life of the average person. A ruler might be good or bad, and that was much more important than whether they were local or foreign. Those higher up the hierarchy would care more whether they were the same nationality as the people over them, but it still wasn't as important as how they were treated.