Are Russian forces getting ready for war in Ukraine? That is certainly the fear among Western leaders and in Ukraine. It was only seven years ago that Russia seized part of southern Ukraine and backed separatists who started a conflict in large areas of the east. Russia is threatening military measures and the US says if it invades then it will hit back with sanctions on an unprecedented scale, so what is going on? Where is Ukraine? Ukraine shares borders with both the EU and Russia, but as a former Soviet republic it has deep social and cultural ties with Russia and Russian is widely spoken there. Russia has long resisted Ukraine's move towards European institutions. When Ukrainians deposed their pro-Russian president in 2014, Russia seized and then annexed the southern Crimean peninsula from Ukraine and Russian-backed separatists captured large swathes of Ukraine's two eastern regions collectively known as the Donbas. Moscow is now demanding guarantees that its neighbour will never join Nato, the Western alliance. Is there a real threat of invasion? That conflict in the east continues to this day. Ukraine says Russia has sent tanks, artillery and snipers to the front in rebel-held areas. But it is the Russian forces beyond the Ukrainian border that are of most concern, and Western intelligence services believe they number up to 100,000. There is no sense of an imminent threat - or that Russia's President Vladimir Putin has decided on invasion. But he has spoken of "appropriate retaliatory military-technical measures" if what he calls the West's aggressive approach continues. Russian Deputy Foreign Minister Sergei Ryabkov has warned that tensions could lead to a situation similar to the 1962 Cuban missile crisis, when the US and Soviet Union came close to nuclear conflict. Western intelligence services as well as Ukraine's think an incursion or invasion could happen some time in early 2022. "The most likely time to reach readiness for escalation will be the end of January," says Ukrainian Defence Minister Oleksiy Reznikov. US intelligence says as many as 175,000 Russian troops could become involved as early as January, and CIA Director William Burns believes President Putin "is putting the Russian military, the Russian security services in a place where they could act in a pretty sweeping way". It could just be posturing in an attempt to get Nato away from Russia's back yard. We've been here before, in April this year, and that time Russia shrugged off smaller-scale troop movements as exercises and then pulled back (although some experts suggest it was only a partial pullback). No obvious concessions were made. President Joe Biden and Mr Putin had a video call on 7 December in a bid to lower tensions, but there is little sign of that yet. What does Russia say? Russia initially described satellite photos showing troop build-ups in Crimea and not far from eastern Ukraine as alarmist. Armed forces chief Valery Gerasimov even said "information circulating in the media about the alleged impending Russian invasion of Ukraine is a lie". But President Putin has since threatened to "take adequate military-technical response measures and react harshly to unfriendly steps". Moscow has accused Ukraine of building up half its army - some 125,000 people - in the east, alleging that Kyiv is planning to attack areas controlled by Russian-backed separatists. Ukraine says that is nothing short of "propaganda nonsense" to cover up for Russia's own plans. Russia also accuses Nato countries of "pumping" Ukraine with weapons. Accusing the US of stoking tensions, Mr Putin said Russia had "nowhere further to retreat to - do they think we'll just sit idly by?" Russia's counter-claim could become a justification for military action. Vladimir Dzhabarov, number two on Russia's Federation Council's international affairs committee, said in early December some 500,000 Ukrainians in the rebel-held areas now had Russian passports. If rebel leaders appealed for Russian help, "of course, we cannot abandon our compatriots", he said. What does Russia want? President Putin has warned the West not to cross Russia's "red lines" on Ukraine. So what are those red lines? One of them is stopping Nato's expansion any further to the east, which includes Ukraine and Georgia. Russia also wants Nato to abandon military activity in Eastern Europe, which would mean pulling out its combat units from Poland and the Baltic republics of Estonia, Latvia and Lithuania, and not deploying missiles in countries such as Poland and Romania. In short it wants Nato to return to its pre-1997 borders. Mr Putin says Russia is aiming to avoid bloodshed and find a diplomatic way out, but that kind of demand is bound to fall flat.