Valerian was defeated and captured whild trying to liberate the
eastern empire from Persian incursions. Defence of the eastern
empire against not only Persians but also Goths was the problem,
which Valerian's failure did not solve.
The capture of Valerian in the Battle of Edessa caused serious
problems for the empire. Valerian was co-emperor in the east and
his son Gallienus was co-emperor in the west. The east of the
empire lost its leader and Gallienus' position of in the west was
weakened.
Following the battle of Edessa, the Persians conducted attacks
in Turkey. These were repelled by Macrianus, a tax collector who
rallied the Roman forces in Turkey. Macrianus and his two sons
became usurpers, taking advantage of the fact that Gallienus was
busy in the west. Macrianus invaded Eastern Europe and the legions
of Pannonia joined his army. Gallienus sent his commander of the
cavalry, Aureolos, with a large force. Macrianus was defeated.
However, this deployment weakened Gallienus' military force in the
west and left him unable to deal with a rebellion in northern Gaul
by Postumus, the commander of the army of the Rhine. Postumus
became a usurper who took Gemania, Britannia Hispania and much of
Gaul and created a breakaway state which historians have termed the
Gallic Empire and which remained independent for 14 years. This
weakening of the now sole emperor also led to a rebellion in Egypt
by an ex supporter of Macrianus. Gallienus had to fragment his
military power further by having to send a force to Egypt to
(successfully) supress this revolt. Gallienus also had to deal with
a large-scale invasion in the Balkans by the Heruli, which was
repelled successfully. Shortly afterwards, Gallienus was
treacherously murdered.
The capture of a (co-)emperor by an enemy was a psychological
blow. It also left a sole emperor overstretched with dealing with
troubles both in the west and the east and a time (the crisis on
the 3rd century) when the empire was subjected to constant
invasions and usurpations. Gallienus was successful at dealing with
the problems in the east, but at the cost of losing a very large
part of the European possessions in the west. It was the3rd emperor
after Gallienus (Aurelian) who restored order in the empire.
