The war between Kiev and Moscow-backed rebels has killed more that 10,000 people, rattled eastern Europe and plunged relations between the West and Russia to post-Cold War lows.
Here are five key facts about the 33-month conflict in Ukraine's mainly Russian-speaking industrial east that has defied repeated international efforts to bring peace:
- Ouster of Russian-backed leader -
The war erupted in April 2014 as the ex-Soviet republic was thrust into a geopolitical tug-of-war between Russia and the West.
It followed the ouster of Kremlin-backed president Viktor Yanukovych in February 2014 after massive street protests over the government's decision to turn its back on closer ties with the European Union.
Moscow responded by annexing Ukraine's Black Sea peninsula of Crimea in March before armed men without any insignia on their uniforms began taking over government buildings across southeastern Ukraine.
Russian President Vladimir Putin later admitted on television that he plotted the Crimean invasion to "save" the Russian-speaking region.
Kiev and the West say Russia also instigated the eastern uprising and poured arms and troops across the border to bolster the two self-proclaimed rebel republics of Donetsk and Lugansk.
But the Kremlin portrays the war as an internal conflict between an ethnic-Russian minority angered by a "coup" in Kiev and a nationalist government.
Putin once said that any Russian citizen captured or killed in the war was a volunteer "following the call of their heart".
- Ukraine forces struggle -
The war began with Ukraine's army in shambles after decades of neglect.
Kiev announced an "anti-terrorist" operation in April 2014 to push the Russian-backed separatist fighters out of towns in the industrial regions of Lugansk and Donetsk.
But government troops suffered embarrassing early setbacks.
Pictures emerged of woefully ill-prepared soldiers simply giving up their weapons and tanks to the rebels when faced by angry crowds of locals.
Kiev often had to rely on a patchwork of volunteer battalions -- which included prominent far-right nationalists -- before co-opting them into a new National Guard run by the interior ministry.
Government forces eventually made gains but the conflict settled into a stalemate. Ukraine estimated it was facing 40,000 insurgents and 10,000 regular Russian forces at its peak.
Kiev has taken some strides towards bolstering its fighting capacity -- although shortages of equipment remain common among troops who are spearheading the fight.
- Economic shambles and corruption -
Heavy military spending and the loss of vital industries have seen Ukraine's economy shrink by about 17 percent between 2014 and 2015.
Annual inflation rocketed to nearly 50 percent last year while the local currency has plunged against the dollar.
Kiev has since negotiated a big debt writeoff deal with private creditors and secured a $17.5-billion (16.4-billion-euro) rescue loan from the IMF and other aid from foreign allies.
But Ukraine has received just a fraction of the IMF cash because endemic corruption remains a hangover of its Soviet past.
Analysts believe the problem has been especially severe in the army.
They say senior commanders embezzle money assigned for new equipment and that lower-level officials accept payments from parents who don't want their children to go the battlefield.
- Western response -
The United States and the European Union reacted to the annexation of Crimea by imposing economic sanctions on Russia and Putin's inner circle.
NATO has also created a spearhead force to ward off any potential Russian advance into the Baltic states and eastern Europe.
Poroshenko wants the sanctions extended for as long as the war lasts -- a possibility that may be in question with new US President Donald Trump seeking to rebuild ties with Russia.
Major Western powers have also flown equipment and training squads into Ukraine to boost the country's fighting capabilities and morale.
But Kiev has been frustrated by the West's refusal to send in weapons that could really counter Russia and swing the war in Ukraine's favour.
- Mysterious rebel deaths -
Perhaps the insurgents' biggest weakness is that they are not a single force and have different agendas.
A string of assassinations of rebel warlords in the east remain unresolved.
Some Kiev analysts believe the killings resulted from a desire of one particular warlord to usurp another while the rebels themselves place the blame squarely on Kiev.
Each of the two republics are seeking full autonomy from the central government and have their own self-proclaimed presidents.
