If Russia and Ukraine slide into outright war, the United States and allies will face tough choices on how to support a friendly state they have no intention of making a full NATO member.
In what appeared to be a dramatic escalation on Friday, Ukraine said it had destroyed much of a column of armored vehicles that had entered its territory from Russia.
Moscow dismissed the account as "fantasy". NATO confirmed what it described as an "incursion". Ukraine did not specify if the vehicles were manned by Russian troops or separatist rebels, and it was not immediately clear if Russia would respond.
If Moscow ever took any such action against a NATO member, that would inevitably trigger the alliance's Chapter 5 mutual defense clause, essentially committing it to war. Ukraine, however, is not a member and Western officials say is unlikely to become one any time soon.
Washington and others have spent much of the last five months chastising the government of Russian President Vladimir Putin, describing its actions in Ukraine as unacceptable. They accuse Moscow of arming the rebels, a charge the Kremlin denies.
The reality, however, is that U.S. and European leaders want to avoid a potentially nuclear superpower confrontation.
Direct military action by NATO states remains entirely off the table, current and former officials say.
"The West is already pushing at the limits of what it feels it can do without finding itself in a serious confrontation with Russia," said Samuel Charap, a former U.S. State Department official, now a senior fellow at the International Institute for Strategic Studies.
"There could be more sanctions, there could be more supportive of Ukrainians, but beyond that I really don't see the U.S. doing too much more."
Non-lethal support - for now
Speaking on condition of anonymity before Friday's incident, U.S. officials said several options were under discussion if Russia escalated further. They include tightening sanctions, freezing Moscow further out of international forums, beefing up NATO's military presence in Eastern Europe and, perhaps, supplying more non-lethal aid.
Sanctions and economic pressure on Russia have prompted capital and expertise flight and a fully fledged trade war - but no policy shift from Moscow.
Critics in Washington and elsewhere say the paucity of the Western response may simply encourage President Vladimir Putin to act further.
Ukrainian forces may have taken heart from the reports of a successful attack on the armored column on Friday. Still, most experts say Russia could outmatch Ukraine if it attacked in force, particularly if it limited its ambition to separatist areas of the East.
A thrust into Ukraine's central and western heartland would more challenging, they say, but Russia would still be expected to win, although with a likely insurgency to follow.
So far Washington has only sent non-lethal items such as body armor - but that could change, officials said.
The dilemma, they said, is that any increase in U.S. military backing for Kiev is almost certainly to be matched by Russia.
At most, any lethal support provided to the Ukrainian military is expected to be limited to relatively small weapons systems such as anti-tank rockets that could help Kiev's forces slow but not necessarily stop a Russian advance.
Western Ukraine peacekeeping exercise
Up to now, officials say the U.S. has avoided sending additional military elements to Ukraine as a matter of policy.
That will change next year if Congress approves a request to send units from the U.S. Army in Europe and/or the California National Guard to Ukraine's peacekeeping training centre in the far west of the country, far from the current conflict.
Increasing training for Ukraine's military in locations outside Ukraine might be another option, although so far there have been no public suggestions of doing so.
Regardless of whether Putin crosses another line, the U.S. officials said more sanctions on Russia are almost certain if he does not end support for the separatists. A key pivot point will be the NATO summit in Wales in the first week of September.
The odds of that meeting producing a significant shift in approach to Ukraine itself seem low, although current and former officials say discussions between smaller groups of member states on what to do may take place on the periphery.
Germany in particular is wary of an aggressive approach while a European arms embargo on Ukraine also limits flexibility.
"It's never easy for NATO as a whole to make decisions," said one former U.S. official on condition of anonymity. "If something is done, it may have to be agreed by a smaller coalition of the willing."