( CNN ) -- When Ukrainian President Petro Poroshenko visits Washington on Thursday , he will almost certainly again ask for U.S. military assistance , including defensive weapons . President Barack Obama should say yes . Arming Kiev can deter Russian Vladimir Putin from further aggression and support the fragile Ukraine ceasefire and settlement process . Doing so would also bolster U.S. efforts to prevent nuclear proliferation . Russia illegally occupied and annexed Crimea in March . Just weeks later , armed separatists -- assisted and , in some cases , led by Russian intelligence officers -- began seizing government buildings in the eastern Ukrainian regions of Donetsk and Luhansk . Ukrainian counterattacks started making headway in the summer . Russia responded by supplying heavy weapons to the separatists -- including , it is widely believed , the Buk anti-aircraft system believed to have shot down Malaysia Airlines flight 17 in mid-July . Despite the influx of arms , Ukrainian forces continued to make progress . By early August , the separatists looked on the verge of defeat . That would have stopped Putin 's strategy in its tracks . So , as NATO noted , elite Russian military units invaded and occupied Ukrainian territory , hitting Ukrainian forces hard . Vastly outgunned , Poroshenko had little choice but to accept a ceasefire on September 5 . The truce is shaky , but the Ukrainian president seems intent on pursuing a peaceful settlement . Whether common ground can be found among Kiev , the separatists and Moscow is unclear . At best , the negotiation will be long and arduous . The Ukrainian government has for some time sought lethal military assistance , such as anti-armor weapons , as well as secure communications equipment and reconnaissance drones . When he hosts Poroshenko in the Oval Office , Obama should agree to provide defensive arms on an expedited basis . Such assistance would enhance Kiev 's ability to deter further Russian aggression , aggression that would threaten , perhaps fatally , the possibility of a negotiated settlement . A better-armed Ukrainian military would give Putin pause , because it could impose greater costs on the Russian army if the Russians break the ceasefire and renew the fighting . Reports detail the extraordinary lengths , such as nighttime burials , to which the Kremlin has gone to hide from its public the fact that Russian soldiers have fought and died in Ukraine . The more costly the Ukrainians can make any fighting for the Russians , the less Moscow 's interest in resuming the conflict . Some may argue that providing lethal military assistance would provoke Putin to escalate . But he has already escalated the situation -- from instigating separatist actions to providing heavy weapons to sending in the Russian army . This suggests the opposite may well prove true : Arming Ukraine will raise the costs of escalation to Russia and thus make it less likely . Putin has made clear his contempt for the Western response to date . Inaction could embolden him to escalate in Ukraine and test the waters elsewhere , perhaps in a NATO member state such as Estonia . Moreover , the Ukrainians will bear the risks and brunt of any escalation . If they are willing to accept those risks , we should give them the tools to defend themselves . And there is an additional important reason to respond favorably to Ukraine 's request for defensive arms : to restore credibility to the notion of security assurances . In December 1994 , the leaders of the United States , Britain and Russia signed the Budapest memorandum on security assurances , which committed those countries to respect Ukraine 's sovereignty and territorial integrity and not use force against Ukraine . Those commitments paved the way for Kiev 's decision to give up about 1,900 strategic nuclear warheads , at the time the world 's third-largest nuclear arsenal . Moscow has grossly violated the Budapest memorandum by its seizure of Crimea and assaults on Ukraine . The United States and Britain have an obligation to respond . Washington has imposed economic sanctions on Russia and provided Ukraine a modest amount of nonlethal military aid . Those are appropriate steps , but they do not suffice . Last Friday , former President Leonid Kuchma , who signed the Budapest memorandum for Ukraine , suggested his country had been cheated . That reflects general sentiment in Kiev . The widely held impression that Moscow has violated its Budapest commitments at low cost badly weakens the value of security assurances in the future . That is unfortunate , as security assurances could play a role as part of the package to resolve the Iran and North Korea nuclear issues . But how much value will security assurances have if Tehran and Pyongyang see that they can be violated with relative impunity ? Washington can change that impression . Providing lethal military assistance to Ukraine could not only deter Putin from further war against Ukraine , but could help to reestablish security assurances as part of the solution to critical nuclear proliferation challenges .
Ukrainian President Petro Poroshenko visits Washington on Thursday . He seems intent on a peaceful settlement , write Strobe Talbott and Steven Pifer . Obama should agree to provide defensive arms on an expedited basis , authors say . Talbott and Pifer : Moscow has grossly violated the Budapest memorandum .