War is always a tad messier than envisioned by the armchair experts who plan them. From Anatol Lieven at responsiblestatecraft.org:
Recent days have seen a new flurry of Western media reports that U.S. intelligence believes that Russia is planning to invade Ukraine early in the new year. These reports have already led to warnings by NATO and Washington that Russia would pay a heavy economic and political price in the event of a war.
Russia, for its part, has denied the dire nature of the reports, blaming “a targeted information campaign,” according to Dmitry Peskov, President Vladimir Putin’s spokesman, on Monday. “This is building up tension.”
Nevertheless, this should also lead to a determined and sincere new effort by the United States and leading European governments to find a reasonable compromise with Russia over the Ukrainian disputes. For quite apart from the global economic damage that would result from a war in Ukraine, and the ways in which China would take advantage of such a crisis, the West has a very strong reason indeed to avoid a new war: the West would lose.
Intelligence about Russian plans may be exaggerated, imagined or even fabricated. Recent years have seen a series of such scares that turned out either to be unfounded, or to be Russian warnings to Ukraine against an attack on the Russian-protected separatist Donbas region of Ukraine.
For Russia, there are massive disincentives against an invasion: The European Union would impose greatly intensified sanctions that would do vast damage to an already troubled Russian economy; the Nord Stream gas pipeline would be abandoned; Russia would be forced into almost complete dependence on China; parts of the Ukrainian army would fight very hard, and might inflict heavy Russian casualties; and if it occupies large new territories, Russia would face the challenge of ruling not the pro-Russian populations of the Donbas and Crimea, but significant numbers of infuriated and rebellious Ukrainians.