(CNN) Tension is once again growing over Crimea, with Ukraine ordering its troops to be on the "highest level of combat readiness."
The order came after Russia accused Ukraine on August 10 of launching a militant attack at "critically important infrastructure."
It marks another fraught moment in relations between the two nations which have been greatly strained since Russia annexed Crimea in 2014.
A NATO official told CNN that it is deeply concerned about the recent upsurge in violence in eastern Ukraine and called on Russia to reverse its "illegal and illegitimate annexation of Crimea."
So what has caused the latest spat and what will happen next? CNN's Nick Paton Walsh answers the key questions.
Are things coming to a boiling point?
It's always very hard to tell. The Kremlin likes to keep its intentions that way. But certainly we've seen months of reports of a military build-up of armor in separatist areas, fomented by a recent spike in clashes along the border between separatist areas and Ukraine's front line with them.
These spikes happen periodically but this week the Russian security services -- the FSB -- claimed to have disrupted a plot by Ukrainian military intelligence to attack inside the Crimean peninsula that Russia controls, and that two of their servicemen were attacked in a failed Ukrainian incursion.
Ukraine says the plot claim is nonsense. But Putin has used it to declare further peace talks as "pointless," and Ukrainian President Petro Poroshenko has put his military in the east on high alert. It's not been this bad since open warfare in 2015.
Why now?
Outside of the Crimea incident, critics of Russia say it has long wanted a land bridge between the areas of Donetsk they control and
Crimea. Sustaining Crimea has proven difficult as they can only supply it from the Russian mainland using ferries.
If you buy into the idea, that this land bridge has always been Russia's plan, why now? Well, Russia has involved itself in another conflict the West needs an end to -- that in Syria. It is perhaps a higher priority to the White House than Ukraine, given the links to ISIS. The
Kremlin may think it has helpful leverage there to increase its chances in lessening Western sanctions if it moves against Ukraine.
Coupled with that, Ukraine's not been handling itself that well.
Nationalists form an increased part of the frontline ranks, and sentiment has gotten uglier against the media too, leading one Ukrainian official to quit after details of journalists who worked in separatist areas were leaked and published online.
Its European allies have grown impatient with the pace of reform. It all leads to more rather than fewer questions about how much automatic support Kiev can expect if its front lines blow up again.
Has the power shifted?
Not really. Russia always had a vastly superior army backing up and often fighting for the separatists. Ukraine has improved its military but remains underfunded and without the Western support it wants. There have been some assassinations of leading separatist figures and failed attempts in separatist ranks lately, which observers have interpreted as actions to consolidate Moscow's control.
The separatists have had over a year to boost ranks and equipment since their last clashes in Debaltseve -- a frontline city of eastern Ukraine -- and Russia is a master of the long game.
Ukraine has been too swamped in chaos to have kept up as an adversary.
Who are the main players?
Putin calls the shots and separatist leader Alexander Zakharchenko carries them out. Poroshenko on his side will face increased pressure not to yield, with a strong nationalist sentiment you may hear voiced through former POW Nadia Savchenko.
Is anyone doing anything about it?
Western sanctions against Russia over Crimea stay in place. So will those over Donetsk. But Russia seems to be weathering the impact domestically, blaming the economic damage on a Western conspiracy. Observers from the Organization for Security and Co-operation in Europe (OSCE) have had their powers limited and much of the fighting happens at night when they aren't around. So, in short: no,nobody's doing much about it, hence why it's slipping so fast.
This is a land war on the European continent involving a nuclear power, with a serious grievance against NATO. The stakes are very high, even though the West's desire to get involved is low.