However, even if Ukrainian troops are unable to repel a large-scale invasion, defence analysts believe Moscow would then find itself in a costly and prolonged guerrilla war that could in the long run turn victory into defeat.

A full-on assault of Ukraine, with Russian forces attacking from the north, east and south, would aim to encircle the most potent arm of the Ukrainian military in the east, as well as make a move on the capital.

Putin launches 'military offensive' - Ukraine crisis latest

How Putin could launch a full-scale invasion

This graphic posted last week by the UK's Ministry of Defence offers an idea of what it described as President Putin's Possible Axis of Invasion:

"The expectation is that Russia could take Ukraine without much of a fight," said a Western source, referring to any initial assault.

This would not be because of a lack of will to defend their land on the Ukrainian side.

But Russia's military doctrine prescribes achieving surprise and shock; maximum use of all lethal force available; rapid tempo of operations; and quick achievement of objectives.

In other words - any full-scale invasion could be over in a matter of a few days.

Russia has assembled up to 190,000 forces personnel, including ground, sea and air, around Ukraine, armed with fast jets, artillery, warships and tanks.

It also has sophisticated jamming technology to disrupt Ukraine's ability to fight back and communicate.

Cyber attacks will likely play a part in any offensive such as by targeting critical infrastructure like power supplies.

Putin's pretext for an invasion

President Vladimir Putin has also spent the past few days setting the stage to fabricate the pretext for what is already unfolding at a dangerous speed.

First there came the manufacturing of a critical emergency in two areas of eastern Ukraine that have been under the control of Russia-backed separatists since 2014.

This included a rise in shelling over the line of contact, with separatists significantly ramping up their attacks against Ukrainian positions using heavy weapons like artillery, while at the same time falsely blaming the increased violence on the Ukrainian military.

Then there was the announcement of an "evacuation" of civilians from separatist-controlled Donetsk and Luhansk regions followed by the dramatic decision on Monday by the Russian President to recognise the two "republics" as independent, in violation of international law.

Where will Russia target?

Military analysts believe the Russian invaders would seek to encircle Ukraine's most battle-hardened forces in the east, bomb their main command posts in and around the capital and disrupt supply lines.

That would severely hamper the Ukrainian ability to respond in time.

Follow the Daily podcast on Apple Podcasts, Google Podcasts, Spotify, SpreakerAirstrikes aimed at key targets in Kyiv, such as the headquarters of the military and the security and intelligence agencies, would aim to destroy the ability of Ukrainian defence chiefs to command their forces effectively.

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It would also try to "decapitate" the political leadership - something that Russia would most likely hope to achieve before it looked to de-escalate and put in place a more pro-Russian administration.