Russia is reinforcing a prospective invasion force that now encircles Ukraine's borders. Evincing the Biden administration's concern, U.S. military ground radar and signal intelligence aircraft were flying off Russian-occupied Crimea on Monday.

U.S. and Ukrainian officials suggest that an invasion may occur by late January. But there are reasons to believe that an offensive may come even sooner — perhaps before Christmas.

Vladimir Putin's interest in attacking Ukraine will be shaped in significant part by the outcome of his video call with President Joe Biden on Tuesday. Putin wants Biden to pressure Ukraine into recognizing the political legitimacy of Russian-supported rebel forces in southeastern Ukraine. He is equally determined to corral Biden and President Volodymyr Zelensky against strengthening Ukraine's links with the West.

That's not all Putin wants, however. Emboldened by Biden's weakness on other Russia-related concerns, Putin is upping the ante. In striking language last week, the Russian leader demanded that Biden provide a new commitment not to expand NATO. This would restrict the democratic sovereignty not just of Ukraine but of non-NATO, Russia-facing nations such as Finland, Sweden, and Moldova. Putin also wants Biden's pledge not to match Russia's development of intermediate-range missiles.

These demands should be utterly unacceptable to Biden, as they would significantly weaken democratic security in Europe during the very same week as Biden's democracy summit. They would also legitimize Putin's effort to turn Ukraine into a vassal state.

Biden should instead warn Putin that any attack on Ukraine will cause Russia's exclusion from the global financial system and new sanctions against Putin's oligarch class, including in the United Kingdom. He should also make clear that it would end Russia's energy export market, including Nord Stream 2. Those energy sanctions might earn support from new German foreign minister Annalena Baerbock (who takes office on Wednesday), but they can be made unilaterally if necessary.

If Biden avoids appeasing Putin, the Russian leader may feel pressure to act quickly. Inflation is rising in Russia, with political pressures likely to grow as food becomes more expensive. A subjugated Ukraine would serve both Putin's strategic narrative and Russian domestic pride.

As for the military considerations, it would be nonsensical for Putin to give the West and Ukraine advance warning of his attack. Putin's prior conduct suggests he would instead retain the element of surprise as to the timing and form of attack. Indeed, the erratic movement of Russian armored vehicles, tanks, and artillery across central and southwestern Russian railways seems designed to preserve this interest.

But there's also another factor that might persuade Putin to attack before the new year — namely, the intersection of Christmas and the weather. By Dec. 21, Ukrainian water features should start to freeze. The ground that Russian armored and mechanized forces would have to traverse will also be especially hard and thus accessible. Putin may wish to take advantage of this optimal window so as to mitigate the risk of his forces still fighting when Ukraine starts to warm up in March. Nor is Russian army morale, discipline, and readiness well-served by keeping very large numbers of soldiers forward-deployed but idle.

At the same time, although Orthodox Christmas (as observed by most Ukrainians) falls on Jan. 7, Putin knows that U.S. military, intelligence, and political activities will slow down over the Gregorian Christmas period. He may sense that a Christmas/pre-New Year offensive gives him the prospect of evading a speedy U.S. response to any offensive. The same principle applies even more so to European capitals, offering the opportunity to slow down Western coordination over any response.

Ultimately, only Putin knows what he'll do. But it would be folly to expect him to be predictable.