Could Ronan have defeated Thanos if Ronan was armed with the Infinity Gem of Power? Officially, we will never know. Ronan the Accuser was destroyed by the Power Gem in 'Guardians of the Galaxy' (2014).
- HOWEVER: With that answer officially out of the way, let us speculate based on their canon comic iterations as the primary source material by which an answer COULD be made.
For the impatient, I will save you the time and assure you, even armed with an Infinity Gem of Power, Ronan the Accuser would have NO CHANCE against Thanos of Titan. Thanos took all of the Infinity gems from their previous owners, save Adam Warlock in the canon Marvel Earth-616.
- During the Blood and Thunder storyline, Thor Odinson gets the Infinity Gem of Power and goes on a rampage across the Marvel Universe. Thanos and Thor mix it up in Silver Surfer Vol. 3, Issue 88. Thor, who is significantly more powerful than Ronan with the Universal Weapon, was now enhanced further with the Power gem. Thanos was unimpressed.
Thor plus Power Gem should equal oblivion for anyone on the receiving end of Mjolnir. Thanos has a bloody nose and smiles. Scary.
Given a little background between the two characters reveals the 'tale of the tape'. Ronan is a man (or in this case, a Kree), Thanos is nearly a god (or god-like being of immense physical power). Given the range between where the two started, Ronan never had a chance.
Ronan the Accuser in the canon Marvel Universe, Earth-616 is one of the finest warriors of the Kree Empire. An empire of thousands of worlds taken in militaristic conquest by a species also genetically experimented on by the Celestials. Residing in the Magellenic Cloud, the Kree were one of the great races of the Milky Way galaxy, contested by their greatest rivals, the shape-changing Skrulls and often in contention with the Shiar and the reptilian Badoon.
The Kree are a dimorphic race, coming in two colors, one blue-skinned, the other classified as pink-skinned. Externally, the Kree resemble Humanity, with bilateral symmetry and opposable thumbs. Internally, the Kree are stronger, denser and physically more resilient than humans, being capable of lifting five tons without any technological aid. Their senses are sharper than a Human's and their intellectual capacity is on par or perhaps slightly greater due to their technologically-advanced society.
Despite the Kree's evolutionary advantages over Humanity, the Celestials considered them an evolutionary dead-end: they had developed their physical and mental capacities as far as they could go and had removed any genetic variability from the species. The Kree were as perfect as a humanoid could be and had reached an evolutionary plateau.
To augment themselves further, the Kree would use technology to improve their physical and mental abilities via cybernetic implants and exo-armor technologies. The best of the Kree warriors used armored suits which could increase their lifting strength from five (5) tons to ten (10) tons. Their armored suits gave them resistance to energy weapons, enhanced senses and the ability to interact with Kree technologies.
The most elite Kree warriors would become Accusers and be assigned an exo-skeletal armor capable of lifting eighty (80) tons, and enabling the Accuser of projecting powerful blasts of electromagnetic energy.
The elite class of warriors called Accusers were considered to be the pinnacle of Kree fighting ability. Relentlessly trained, cybernetically enhanced, and given access to the most powerful of Kree multi-weapons, The Universal Weapon. This device was capable of energy blasts, force-fields, matter manipulation and flight. The weapon has a built-in fail-safe: only Accusers in their armor can safely wield it.
Ronan was the elite of the elite. He was the most famously known of the Kree Accusers and was considered to be one of the finest warriors the Kree ever produced. His reputation as a tactical and strategic genius was unmatched in the history of the Kree. When the Kree needed to defeat an enemy or achieve a victory, Ronan was undefeated.
Given such an elite pedigree, thousands of battles, incredible weaponry and the might of the Kree armada behind him, Ronan proved all but undefeatable until the fall of the Kree Empire by the Annihilation Wave and the subsequent rule of the Kree Empire by the Inhumans.
The Mad Titan, Thanos
Thanos was a member of a humanoid species called the Eternals. The Eternals were also a project of the Celestials designed to bring forth metahuman potential (for a still unknown reason). Thanos was the son of the leader of the Titanian sect of the Eternals. A schism had formed between the Earth Eternals lead by Zuras and the Titanian Eternals lead by Mentor. The group of Eternals lead by Mentor, left Earth and using their powers, built a planetary colony on Saturn's moon of Titan.
- The Eternals appeared to be similar to humans, bilaterally similar with opposable thumbs. The resemblance ends there. The Eternals are capable of absorbing cosmic energy within their cells and releasing it in various ways. At a rest state, all Eternals can lift ten tons without effort. Their bodies are far stronger, faster, more resilient, and heal faster than a human's even without the use of their cosmic energy. Their senses are sharper and more acute than a Human's and their mental capacity is in far excess of the average Human.
In addition to their natural resilience, all Eternals may channel cosmic energy for superhuman feats. Each Eternal may have different levels of capability depending on their personal training and preferences. These feats include super-speed, flight, increasing invulnerability, strength enhancement, energy projection at range, mental powers and matter manipulation. The most sophisticated and well trained with these powers may use all of them with equal facility. Being such as Mentor and Zuras, have few equals among the Eternals utilizing these abilities.
While the range of abilities for each Eternal varies widely, they have proven themselves to be a match for most modern metahumans on the Marvel Earth, with speed and strengths as great as Thor, Iron Man and for brief periods, the Hulk. The versatility of their powers has even secured one or two of them honorary roles among Earth's mightiest heroes, the Avengers. See: Circe.
Of all of the Titanian Eternals, none are the equal of the most powerful of them, often called The Mad Titan, Thanos. Thanos is a mutant among the Eternals, born with Deviant Syndrome increasing his physical abilities and his mental instability. Thanos has always had a lust for power and has expanded his abilities far beyond his brethren. His physical strength is unmatched by the Eternals. He can lift one hundred tons or more. He has stood toe to toe with Thor, Iron Man and the Thing simultaneously and easily held his own. This is without using any form of augmentation or manipulation of cosmic energy.
He is also believed to have been further enhanced using alien cybernetic technology giving him even greater physical durability and resistance. He has taken blows from Thor's hammer, Mjolnir, and was unfazed.
While Thanos rarely uses his energy projection abilities, they are quite formidable. He is able to project energy from his eyes and sheath his hands in an energy field capable of destroying any matter it touches.
Despite his incredible physical capabilities, Thanos' greatest asset is his unmatched intellect. He has scoured the galaxy learning whatever he could from whomever would teach him. He is a master of alien technology which has allowed him to battle against the Universe's mightiest beings such as the Stranger, the Elders of the Universe and even Galactus. His genius-level intellect has even tapped into mystical powers allowing him to commune with forces such as Eternity and to woo the Conceptual Entity known as Death.
While the Thanos of the Marvel Cinematic Universe will not likely have this ability, in the comics, Thanos is bared from Death's realm, rendering him functionally immortal. Granted he was already extremely long-lived as an Eternal, but currently, he cannot die by any means.
If these two men were to meet, despite his strategic and military training, Ronan would be undone before he began.
Thanos outmatches Ronan in pure physical ability. Even armed with the Universal Weapon, Ronan alone cannot do enough damage to Thanos to gain the upper hand. Thanos, even without using his innate energy powers to improve himself is incredibly damage-resistant. It doesn't mean he can't be hurt. It just means he won't stay down.
The infinity gem might make this more challenging since it would augment Ronan's power considerably, but since he is barely able to control the gem, probably due to his armor's protective properties, Thanos would be focusing his attention on said armor. Ronan is dependent on his technology to win against Thanos. The Mad Titan knows this and would focus his efforts on removing Ronan from his technology.
Thanos does not fight fair and is known to use deception to gain access to what he wants. He manages to gain all of the Infinity Gems from the Elders of the Universe (powerful last survivors of various species who maintain their existence by preoccupying themselves with their obsessions) by threat, deception or good old-fashioned brute force.
He never gives up an advantage to an enemy unless he already has a plan to defeat that enemy. Hence, even if Ronan ended up with the Infinity Gem, it was because Thanos allowed it and already knew of a way to counter it.
Thanos is a Magnificent Bastard, trickster, chessmaster, manipulator; he plays the game of power acquisition at a level rarely seen in the Marvel Universe. Thanos is the Eternal who managed to gain the power of the Infinity Gems, not once, but twice and managed to gain control of technology capable of destroying all of the Marvel reality on at least three separate occasions.
Ronan is powerful, he is relentless, he is formidable. In the end, he is still just a Kree. Augmented by nigh-invincible technology, backed by one of the most incredible fighting forces in the Marvel Universe. Against an alien army, Ronan the Accuser is the Kree to fight alongside.
But against Thanos, he is a man fighting against a force of Nature. Thanos is a being whose very willpower drove him to destroy half the Universe to get what he wanted.
Ronan never had a chance.
Jim Starlin makes this point clear in the graphic novel Thanos: The Infinity Revelation when Thanos beats Ronan handily, barely breaking a sweat.