This almost seems silly.
Quick look at the Challenger II's weapons:
- Main cannon
This is of lesser use as it's ultimately an anti-tank weapon and we're taking it to times where anti-tank isn't really a requirement. This would have some pretty impressive effects when targeting castle walls. The range on this puts any form of ancient artillery to shame and it's accurate enough that targets like 'enemy general' could easily be obtained and eliminated with a high explosive round. Limiting factor here is 49 rounds is the standard carry out
- L94A1 chain gun
This is the silly weapon. Up until the development of tanks, there is very little out there that has the ability to stop one of these rounds. Enemy infantry and cavalry would be mowed down incredibly quickly. Carries some 4200 rounds.
- L37A2 (GPMG) machine gun
This is an optional mounted remote device, fits into the category above.
Range... I doubt roads are too considerable, at least not ones this tank wouldn't rip to pieces. Gives it about 250km.
I'm actually having problems finding an example of a single battle that this tank wouldn't have a profound impact on up until the 1900's. There are a few where the number of troops involved is high enough that the tank itself wouldn't win the battle, but the tank would skew the battle heavily in a few manners. First is morale: the effectiveness of this tank is such that it would induce heavy fear in an enemy (and probably be referred to as magic). Second is its ability to launch a precise and deadly strike from a huge distance, if this is used to hit strategic targets (enemy generals for example), then the tank could cause such chaos that it could potentially change the tide of a battle (or war) from a single well placed shot: figure out what section of a castle houses the king and take it out with a single well placed blast from the main cannon.
Imagine a Roman legionairre in Testudo formation: they are heavily shielded and nearly immune to enemy fire. Immune atleast until their unit is hit by a single high explosive round from 5 km away that obliterates the unit. Imagine being in the Testudo next to the one that just went up in a fiery hell and tell me you wouldn't drop your useless shield and run.
The largest cavalry charge in history (in a single charge) was (according to wiki anyway) 3000 heavily armoured polish lancers and german/austrian knights towards an ottoman force during the siege of Vienna. This single tank would then have to average 12 bullets per kill to eliminate the charge entirely by itself with just its machine gun (add in as per comment, I don't think the main battle cannon would really be needed vs cavalry). Actually, the size of the forces involved in the battle of Vienna might be high enough that the tank wouldn't be able to win the battle singlehandily, but it could have stopped the charge that routed the ottomans in its path and changed the outcome of that battle (if the ottomans had the tank to open up the walls at vienna, the city would have fell long before the charge happened anyway)
But you mention war and not a single battle. If you consider the Golden Hordes invasion into Europe a single 'war', then this tank might not have had that much of an effect simply due to its range and ability to get into a battle. Mongols on horseback could out maneuvered the tank on these vast plains and left the tank unable to directly engage and involve itself in a fight.
The Punic wars between Carthage and Rome was heavily fought at sea, so there might be a pretty ready instance where one side having a tank might not have a huge advantage. If the tank was parked in Carthage, it could have prevented the Romans from ever sacking the city of Carthage, however the Romans could have annexed most territory (Iberian Spain and Mediterranean islands) but never actually take Carthage. I guess there is technically an instance where the tank might not have impacted the outcome of the war?
If the Aztec possessed the tank, they could have really easily stopped one of their great cities from being razed by the Spanish, but the spanish ships could have easily avoided the one city that possessed the tank and hit other ones instead. Does this count as not being able to impact the outcome of a war?
Just to add: The potential of the tank and its main cannon to become a battlefield assassin of sorts would immediately impact history. Napoleon is a safe 5km away from the frontline and giving his orders, until a high explosive round comes from 8km away and ends the general. Imagine if General Lee on the first day of his Seven Day Battles rode up onto a hillside to survey the battle before him and was struck by an incendiary round. Generals in modern warfare vs tanks do not take front row roles in battle just for this reason: this tank engaging in any battle where the general is actually on the field has pretty much free ability to snipe off a famous general
added to the comments: I don't believe there is a reliable method prior to WWI where this c2 tank could be disabled and taken out of combat that isn't related to the C2's supply chain. If you assume this tank is part of an army that it's supporting, then the tank will have some degree of protection on its flanks or when its crew is resting. Pre-1900 a tank wasn't known and weaknesses such as targeting the treads wouldn't be well known (and even then, the c2 has most of its treads armoured and not easily seen). There is the potential for a lucky cannon shot, but the C2 completely outclasses these cannons for range and accuracy so the opportunity for the lucky cannon shot is minimal at best. I'd suggest a 1600's cannon fire could directly strike the c2 at a 1km range and do minimal damage at best, maybe a loud clang?
The first strong resistance this tank could potentially meet is WWI: by then a tank wasn't a completely foreign concept and a few of its limitations/weak spots could be known. Additionally, the armaments this tank has were in use by WWI (admittedly in their infancy), which means opposing soldiers are already expecting machine gun fire and heavy cannon fire and are taking means to protect themselves from it such as digging in and spreading out, unlike previous conflicts where this tank could just open up with its machine guns into the middle of tightly packed troop formations that have no clue whats coming at them nor how to dig in and defend vs it. I would also suggest that WWI artillery was in heavy enough use that enough concentrated fire from artillery and mortar rounds would be the first counter that could potentially disable this extremely advanced tank from a combat point of view
Contrary to Guildsbounty, I don't agree that this single tank would be a game changer in WWII, except in some very specific scenario's. WWII saw much street to street fighting and in an urban environment the possibility of infantry attacks that knew how to disable a tank (sticky bombs) was a much more real possibility. Even artillery would stand a chance in disabling this C2 during WWII. Later in the war, a few T-34's using ramming tactics, or a direct strike from a couple of Stalins Organs (Katyusha rockets) could put a relatively quick end to this advanced C2 tank.