Bibliothèque universitaire de la Défense

Ukraine’s offensive force consists of at least a dozen brigades (some sources say up to 18), nine of which have been armed and supplied by Western allies (a brigade tends to be several thousand men). Those nine are due to have more than 200 tanks, 800 other armoured vehicles and 150 pieces of field artillery in total, according to American intelligence documents which leaked onto the internet in early March and widely circulated in April. It is a large force, but with some glaring weaknesses.

The majority of its vehicles are unarmoured. The amount of artillery is relatively modest—the 21st brigade appears to have just ten guns allocated to it. Notably, the newest equipment is spread thinly across units rather than concentrated in a few. Ukraine might make changes to its order of battle in response to the leaks, but it cannot dismantle and reconstitute brigades that might have been training and preparing together for weeks or months.

One problem for Ukraine is how to achieve surprise. If it masses its forces at a particular spot, Russia might detect those preparations and shore up its lines accordingly. That puts a premium on deception, notes Mick Ryan, a retired Australian major-general. Ukraine will have to conceal troop concentrations, artillery positions, headquarters and logistics hubs. “It could also mean we will see lots of smaller mini-offensives rather than a couple of large ones,” says Mr Ryan, “just to confuse the Russian targeting cycle and to deceive them about Ukraine’s main effort.”

If Ukraine can achieve surprise, the next question is whether it can punch through Russian defences and send more forces quickly through the gap. It will need mobile air defences to keep Russian planes at bay; it is not clear if it has enough. It will have to cross rivers and minefields—obstacles that have consumed entire Russian brigades in the east—as well as a formidable network of Russian trenches and fortifications (see map). “There is no military endeavour that is more difficult to plan, orchestrate and execute than combined-arms obstacle breaching,” says Mr Ryan.

In theory, precision artillery can quickly take out prepared defences, says Ben Barry of iiss, a think-tank in London, pointing to Britain’s use of such systems to destroy bunkers in Afghanistan. But that requires expert synchronisation of artillery, infantry and armour so that troops advance neither too early, while the defences are intact, nor too late, when Russia’s rear echelon has reinforced a barrage’s site.

To date, Ukraine’s army has largely conducted sequential operations—first artillery fire, then a ground advance—rather than these more demanding co-ordinated ones, says Franz-Stefan Gady, a military analyst. That is in part because of rigid Soviet-style commanders and a lack of combined-arms training at scale, he says. Improving Ukraine’s command and control has been a priority for Western officials helping to train and advise Ukrainian generals in Germany over recent weeks.

The timing of an offensive is also uncertain. Weather is one factor. American intelligence analysts reckon that the ground in eastern Ukraine will remain muddy until early May. Kit is another. A third of the Western-supplied brigades are not due to be fully equipped and trained until the end of April. Ukraine’s general staff could launch a staged attack, with some brigades thrown in later as they arrive, but might choose to “save it all up for a big bang”, says Mr Barry. This could maximise the pressure on Russian defences. Waiting for too long could also let Russia dig in further and replenish ammunition.

Western officials familiar with Ukraine’s preparations are unsure how everything will pan out. It is vital, they say, that Ukrainian forces have the confidence to keep moving forward. Russia’s layered defences are designed to lure advancing columns into “kill zones” covered by pre-sighted artillery. If troops panic and freeze up, they could be decimated. But there are also concerns about the opposite: an unexpected collapse of Russian forces that puts Ukraine’s army at the edge of Crimea, in a position to seal off the peninsula, attack Russian ports and bases there and deny the Sea of Azov to Russian ships. Large pockets of Russian troops could also be trapped in Kherson and Zaporizhia oblasts.