(CNN) Heavily armed Russian troops are pushing rapidly towards Ukraine's capital of Kyiv, and US officials are warning the city could fall within days.
The military capabilities of these two nations are so imbalanced that Ukraine's defiant president, Volodymyr Zelensky, implored his Western allies overnight to do more than impose sanctions to get Russian troops off Ukrainian soil.
Here's how the two country's military capabilities stack up.
Just a look at the amount of money the two nations spend on defense gives an indication of the gap between the two. Ukraine spent $4.7 billion in 2021, just over a tenth of nuclear-armed Russia's $45.8 billion, according to "The Military Balance" report by the International Institute for Strategic Studies (IISS), published last week.
While the Russian government launched a drive in 2008 to modernize its military, after a brief war with Georgia laid bare equipment shortfalls, Ukraine's weapons are still largely from the Soviet era. In its ambitions to join NATO, however, Ukraine has reorganized the structure of its armed forces, and has plans to re-equip its air force with Western combat aircraft by the mid-2030s. It also plans to improve its naval capability.
Where Russia has 900,000 active personnel in its armed forces, and 2 million in reserve, Ukraine has 196,000 and 900,000 reservists. Ukraine on Wednesday began conscripting reservists aged 18-60, according to IISS.
In land forces alone, Russia has a twofold advantage, with 280,000 troops to Ukraine's 125,600. And its air force is nearly five times as strong, with 165,00 to Ukraine's 35,000.
But in terms of how many troops are in this particular operation, Yohann Michel, a research analyst who worked on the IISS report, said Russia had an estimated 200,000 personnel in and around Ukraine.
"That's including around 60 battle groups. The situation is evolving rapidly and that number could change, but it's very large, and that's important. It's one of the largest in eastern Europe that we've seen in years -- in my lifetime," Michel told CNN.
"Ukrainian numbers are more difficult because everything they have is now mobilized and they have called in reservists."
There is a lot to compare here -- attack aircraft, armored vehicles, surface-to-air missiles -- but in general, Russia simply has more of everything.
Russia has more than 15,857 armored fighting vehicles, for example, to Ukraine's 3,309.
It has more than 10 times the aircraft -- 1,391 to Ukraine's 128, and 821 helicopters to Ukraine's 55, if you include Navy aircraft.
And where Russia has 49 submarines, Ukraine has none, according to IISS.
"There are big difference between the two forces -- Russia has a much more capable air domain, its air defenses are stronger," Michel said.
"There is a huge difference on helicopters and planes available at every level -- from transport aircraft to fighter aircraft to attack helicopters."
Ukrainian forces have, however, launched some successful counterattacks.
Michel notes that the sheer combat will among Ukrainian troops to fight has been the armed forces' greatest strength.
"I'm really surprised at what I'm seeing. The fact that Ukrainian fighter jets are still flying, they actually took off is impressive."
If Russia takes the capital soon, it will be difficult for Ukraine to fight back. But if it is unsuccessful, the conflict could become protracted. And that's the scenario where Ukraine's will may prove a powerful weapon, Michel said.
But with fighting going on across the country, the number and quality of armory is a major factor.
Nick Reynolds, a Research Analyst for Land Warfare at the Royal United Services Institute, said that Ukraine's air force was now largely knocked out.
"There have been serious issues with modernization due to Ukraine's economic problems since 2014, so the military lacks modern air defense and artillery, particularly counter-battery artillery radar and other surveillance equipment to detect enemy artillery and at least stand some chance of suppressing it with their own," Reynolds told CNN.
"These capabilities are critical for Ukraine's army to be able to maneuver and resupply itself."