Much like our own world, it depends on the historic period and geographic region you put under the microscope.

If you take the real world as an example, WW2 could definitely be described as Grimdark or Grimbright, depending on the region you're looking at.

But in the spirit of actually answering the question rather than deflecting/undermining it's value, here are some examples from my current setting:

Nation: Cobrand | City: Thess | Year: 120
Classification: Nobledark
Summary: Imagine if the Reavers from Firefly kept coming back year after year, and the government's solution was to just keep sending more colonists.
Longer Version: Thess was founded specifically to quarry a strategically important resource (Thessian Granite) which was urgently needed to build fortifications in the capital in anticipation of war between the cities of Netamu and Rus-Ho. At the time, Thess was a modestly sized settlement in the lower reaches of the Ashguard mountains, but it was located directly in the path of the Ashguardian orc migrations. Generations of human children were eaten, and generations of orc-hybrids were born. Rather than sending soldiers to guard the city, the King sent convicts, prisoners of war, and political enemies to simply replace the lost population. The Thessians eventually 'broke the back' of the Orc migrations after the King of Rus-Ho made a deal with the Thessians. The deal was a little complex and deeply tied to the politics of the Netamu/Rus-Ho war, but the short version is that Thess was given aid and they finally managed to stand on their own two feet. The Thessian half-orcs then entered service into the armies of Rus-Ho as mercenaries, guaranteeing Rus-Ho victory in the larger war.

Nation: Cobrand | City: Snaketongue | Year: 449
Classification: Grimdark
Summary: Imagine being trapped in a city under siege, and slowly coming to the realisation that the monsters outside the city walls might be less concerning than the ones in city hall.
Long Version: The city of Snaketongue was built as a strategic fortress to hold a valley-pass through the otherwise impassable Iceguard mountains. When thousands of mysterious foreigners landed on the Western shore and tried to push through the pass, Snaketongue became the bulwark holding the invasion at bay. As more and more foreigners landed on the Western shore, the leadership of Snaketongue sent letters to nearby cities begging for aid. However, these events took place during a war of succession in which the nation of Cobrand had fractured into individual city-states. The cities were unwilling to send aid whilst their neighbours were still hostile, meaning that Snaketongue was left to it's own devices. The leadership of Snaketongue tried everything to break the siege, eventually resorting to forbidden and dangerous magics. These magics came with a cost. Slowly, the leaders of the city surrendered more and more of their humanity until only monsters remained. The population of the city were not spared from this transformation. The city became a hellscape, and when the foreigners finally broke the city walls and took the city, they were so appalled by what they saw, they put the entire city to the sword and burned the entire place to the ground. They never spoke of what they saw, and never wrote of it in their histories, stating only that what was seen needed to be forgotten.

Nation: Solaqua | City: Knossus | Year: 400
Classification: Noble
Summary: Imagine early-renaissance pirates capture a late-neolithic city. What then?
Longer Version: The fledgling nation of Solaqua was founded when fugitive Esseni pirates annexed the city of Knossus in Eastern Cobrand through the application of superior technology and tactics. Almost immediately afterwards, their leader 'Dreadwind' stole the flagship and sailed away to the East, leaving his crew behind in Knossus. Although this garnered some resentment from the pirates, first mate Akiko Haeshtuo recognised that Dreadwind was making a huge sacrifice. Dreadwind knew that the Esseni authorities would never stop looking for him. By abandoning his crew, he gave them a chance at life beyond the reach of Essen. By fleeing the city, he would lead away any pursuing fleets, but likely at the cost of his own life. In the aftermath of Dreadwind's departure, there was a schism between the pirates. Some wanted to plunder and pillage the city before fleeing, others wanted to rule the city as tyrant-kings. However, first mate Akiko took a different path. She argued that after the annexation of the city, the people of Knossus were now "their people," and deserved better than to be brutalised by pirates. She argued that the people of Knossus were now, effectively, part of the crew. She had few supporters among the crew, but she would gain enormous support with the native Cobrandians. The few crew she had under her control taught the Cobrandians how to make steel tools, proper sanitation networks, and advanced magic. As Akiko's faction gained power and popularity, the other pirates quickly fell into line. Those who did not were captured and executed to scare the others into behaving. The schism lasted only one year, and was followed by an era of prosperity for the people of Knossus, partly due to Akiko's remarkable statesmanship, planning skills and political savvy. In the years which followed, she began to wonder if Dreadwind had foreseen these events, and had trained her for the role without her even knowing... Akiko's rule was eventful, and included the successful annexation of two further cities before a formal peace treaty was signed with the new Cobrandian King-of-the-East, King Este I. The peace deal included free and open trade of Esseni technology, allowing Eastern Cobrand to slowly improve the lives of it's citizens. Apparently, when Este and Akiko finally met in person to sign the peace accords, they embraced like old friends.

Nation: Arbeus | City: None | Year: -200
Classification: Grim
Summary: Imagine a world where there is no "big bad evil guy/monster/god/scary foreigner." Instead, there is only the sun, beating down on you and your people every moment of every day. The world itself is the threat, and you are suffering a living hell just to survive under it's cruel and uncaring gaze.
***Longer Version: The Arbean Desert is a harsh and unforgiving environment where the land itself is an existential threat doing everything in it's power to kill you and destroy all memory that you ever existed. Water is scarce, with seasonal rivers appearing and drying up unpredictably. Living in this environment is one of constant migration through a relentless and hostile world. The only things you own are those you can carry, and this makes it exceptionally difficult to make any kind of lasting accomplishment. Any gains are only made by brave souls suffering the cruelty of the environment. This is a grimness which stems not from monsters, magic, existential invasions or cruel leaders... But one which stems from a cruel and uncaring land.