Putin's invasion a blunder of epic proportions, reckons BFBS Forces News Ukraine expert

It's been 1,461 days of brutal war, and still there's no end in sight.

Russian President Vladimir Putin's "Special Military Operation" will go down in military history as a blunder of epic proportions - one that ranks alongside both Napoleon and Hitler's disastrous invasions of Russia.

Some may even draw parallels to the US invasion of Iraq; such was Mr Putin's hubris.

• RAF teaching Ukrainians to become helicopter instructors as Interflex training expands
• Ukraine is far scarier than anything I did in Afghanistan, says ex-Army officer
• Former Conservative MP who fights for Ukraine announces switch to Azov Brigade

Mass, the concept previously downplayed, has returned 

On the battlefield, this war has reinforced the efficacy of artillery and long-range weapons.  

Mass, a concept that just a few years ago was being downplayed by many Western military commanders, has returned. 

The Russian army is slowly gnawing away at Ukraine, not because it has superior doctrine or will, but because, unlike Kyiv or indeed any Western democracy, it is willing to throw hundreds of thousands of young men into the fight without caring if they live or die. 

As General Valerii Zaluzhnyi, Ukraine's former Commander-in-Chief, and now ambassador to the UK said in a speech this week: "This war, long and highly intense [has shown that] the most expensive resource in such a war is human.

"[That's] because it takes too much time to restore it, which will be much longer than the production cycles. Such a resource simply cannot be quickly replaced on the battlefield."

Transformation in air, land and sea domains

The war in Ukraine has also, of course, transformed the air, land and sea domains. 

The explosion in drone technology on both sides means so-called kill-zones grow ever larger - in Ukraine up to 15 miles deep - putting humans further away from the line of contact.

The battlefield is now transparent, which means the number of humans who can physically operate within it - and survive - is dwindling. 

Modern warfare has gone beyond trenches, tank battles and air duels. Semi-autonomous systems, increasingly powered by artificial intelligence, are already appearing in Ukraine. 

Currently, these robots still require human input; the technology is still being developed.

But the move to complete autonomy - thinking systems that learn and make decisions - is coming. 

Four years on, the war in Ukraine is providing an ever-clearer vision of what warfare is going to look like.