For the Mithridatic Wars, here is a direct excerpt from A History of the Armenian People, by George A. Bournoutian, PhD:
"Tigran of Armenia and Mithridates of Pontus realized that Roman
and Parthian presence in the region was a constant danger to their own
sovereignty. Civil war in Rome and problems over the succession in
Parthia, encouraged them to attempt the creation of a third force in
the region, a federation led by Pontus and Armenia, which would
challenge Parthia and Rome. The alliance was sealed by the marriage of
Tigran to the daughter of Mithridates. His eastern flank secure,
Mithridates annexed Cappadocia and the coast of Asia Minor. Parthia and
Rome, realizing that this alliance would be detrimental to their own
designs, agreed to forgo their differences and to concentrate on
eliminating the new threat. This was the first but not the last time
that the two powers would divide the region into zones of influence.
Sulla, who like subsequent Roman commanders viewed a successful eastern
campaign as an opportunity to gain politically and materially, returned
to drive the Pontic ruler out of Cappadocia. In 84 B.C. he managed to
force Mithridates out of Greece and returned to Rome to assume the
title of dictator. Mithridates did not give up his quest, however, and
for the next ten years kept the Romans occupied by invading Greece and
challenging Roman authority in Asia Minor.
With Mithridates keeping the Romans at bay and the western flank
secure, Tigran concentrated on the east. The death of the Parthian king
and nomadic invasions of Parthia from Central Asia, allowed Tigran in
90 B.C. to retake the valleys he had ceded to Parthia; he then expanded
south and took parts of Mesopotamia. By 85 B.C. Tigran began using the
Persian title "King of Kings" and had four viceroys in official
attendance. When a group of Syrian nobles invited Tigran to rule, he
annexed Commagene, northern Syria, Cilicia and Phoenicia. Tigran's
empire thus extended from the Mediterranean to the Caspian Sea, and for
a brief period, Armenia was an empire. Antioch, the great Seleucid city
and capital of Syria, became Tigran's headquarters in the Levant.
Tigran thus took control of much of the former Seleucid territory east
of the Euphrates. To better manage his large empire, however, Tigran
built a new capital, Tigranakert (Tigranocerta), and forced immigration
of Jews, Arabs and Greeks from Mesopotamia, Cilicia and Cappadocia in
order to populate it and other new Armenian cities. Tigranakert was a
great city with walls reportedly so wide that storehouses and stables
could be built inside them. (He goes on to talk about the city, which i
will skip for convenience)
When Sulla retired from public life in 79 B.C., new military
commanders sought to advance their standing. The Roman Senate gladly
authorized foreign campaigns in order to lessen civil unrest and to end
the Mithridatic wars, a thorn in Rome's eastern ambitions. In 74 B.C.
the Roman general Lucullus invaded Pontus and forced Mithridates to
seek refuge in Armenia. Unwilling to break the Armeno-Pontic front
against Rome, Tigran refused to surrender his father-in-law and faced a
Roman attack on Armenia. In 69 B.C. Tigranakert was besieged by
Lucullus. When the city's inhabitants, a majority of whom were
non-Armenians, opened the gates of Tigranakert, it fell to Roman troops
and was looted. Tigran's local governors threw their lot with Rome, and
Tigran lost control of Syria and Mesopotamia. Lucullus tried to take
Artashat but failed, and, when he was unable to form an alliance with
Parthia, returned to Rome. Tigran and Mithridates then began the
reconquest of Pontus, northern Syria and Commagene. Rome did no
surrender its claim, however, and sent Pompey, who defeated Mithridates
and forced him to flee eastward. Pompey then advanced toward Armenia.
Meanwhile, two of Tigran's sons betrayed him, one joining Pompey, the
other the Parthian camp. The Roman presence in Armenia also incensed
the Parthians, who wanted to ensure their control of the lands east of
the Euphrates. In order to end the Armenian and Roman threats and to
regain its territory, Parthia, taking advantage of Armenia's
vulnerability attacked from the east. Tigran resisted the Parthian
attacks at Artashat, but when Pompey arrived, he realized the futility
of resisting the Romans, and in 66 B.C. agreed to the Peace of
Artashat. Pompey, in order to maintain Armenia as a strong buffer and a
friend of Rome, while, at the same time, keeping Parthia in check, left
Armenia intact and allowed Tigran to retain the Persian title, "King of
Kings". Tigran ruled for another ten years and died in 55 B.C. Having
resolved the situation in Armenia, Pompey pursued Mithridates, who
committed suicide on an island in the Black Sea."
Map of Tigran's empire:
I dont know why it shows Armenia as a vassal of Pontus in the map in
Decebal's post. The alliance was jointly led by Mithridates and Tigran,
and as you read, Mithridates would have been long gone if it wasnt for
his alliance with Armenia.