There is no weapon of war that is not vulnerable to another type of weapon: artillery, while devastating against mass concentrations of infantry, is vulnerable to attack from the air. Tanks and armor, which can punch through a front and encircle enemy forces quickly, can still be destroyed by a single soldier armed with a modern anti-tank weapon. Aircraft, which can engage a range of targets on the ground and in the air, can themselves come under fire from enemy fighter aircraft, surface-to-air missiles, or, again, a single soldier equipped with a portable anti-aircraft missile system.
Entire armies too have their vulnerabilities: the German army was unprepared for the Russian climate. The English at Bannockburn were defeated by their own arrogance and overconfidence, and the French army was defeated in a few weeks in 1940 by its own decrepit, incompetent and defeatist generals.
Moreover, all modern armies share a weakness that will cause their defeat if an enemy can exploit it: they are vulnerable to the disruption of their logistics and lines of supply. Without a constant flow of ammunition, food, weapons, equipment and reinforcements, any conventional fighting force will soon grind to a halt. Even if command and control – another prime target for disruption by an enemy – are still fully functional, there is not much a soldier who has no bullets for his gun can do but surrender, no matter what his orders are, once he is encircled by an enemy who has an ample supply of ammunition.
Perhaps the most famous example of the failure to achieve such an encirclement in the history of modern warfare occurred in June 1940 in northern France and Belgium. The Panzer divisions of Nazi Germany punched through the allied (at that time Britain and France) lines in the hilly and forested Ardennes region, which the allies had wrongly thought impassible to a large armored force, and swept headlong west and then north to cut off the French and the British Expeditionary Force (BEF). The allies, who had expected the Germans to take their traditional invasion route through Flanders, moved forward, as according to their plans and expectations, to meet a more conventional and slow moving force (Army Goup B under Colonel-General Fedor von Bock), which was advancing through the Low Countries and was intended by the Germans to draw the allies forward into a pocket that would be closed by the Panzer divisions of Army Group A, commanded by Colonel-General Gerd von Rundstedt, which was already racing around the allies' rear. For a reason that puzzles historians to this day, Adolf Hitler gave the order (or rather confirmed an order given by Rundstedt) to halt the advance of his Panzers at this crucial time (perhaps wanting to give them time to rest and refit before turning south to attack the heart of France, or perhaps to give Hermann Goering, the head of the Luftwaffe, the chance of glory in destroying the allied armies from the air). This gave the British the chance to evacuate the bulk of the BEF (almost 340,000 soldiers) from the port and beaches of Dunkirk, although 35,000 French who were left behind guarding the British retreat were captured. Had the BEF been encircled and trapped in France, the British would have faced a disaster, with no army left from which to rebuild, and Churchill would have been forced to come to terms with Nazi Germany. If that had happened, the world would be a very much different place today.
Looking at a map of the present conflict zone in eastern Ukraine, the rebel forces, as the Allies did in 1940, appear ripe for encirclement. Rebel-held territory extends like a fat thumb into the middle of the Donbas in southern Luhansk and northern Donetsk oblasts, with the base of the thumb being a short stretch of the Ukrainian border with Russia in the east. It is through this border that the rebels have been supplied, for several weeks now, with men and matériel – up to and including armored personnel carriers and even tanks.
The task facing the Ukrainian forces is thus to push along the border, north from Donetsk and south from Luhansk, to sever this thumb from the hand that sustains it. Once cut off from their supplies, the rebel force will start to wither. Ukrainian troops can continue to squeeze the pocket in which the rebels will have been trapped, forcing them to expend ammunition and lives, or they can simply wait for the force to collapse in on itself, and move in to mop up.
It's really that simple. The only conceivable reason that Ukraine has not yet done this is that it lacked a sufficient number of men. But Ukrainian forces are now becoming stronger, while the rebels are weakening. The task of closing the pocket along the border should be given to the regular army, while the volunteer battalions that have been raised since the beginning of the hostilities in the east should be given the job of holding the perimeter around the rebel territory, and perhaps advancing opportunistically as the rebels withdraw and consolidate (as they inevitably will have to as their supplies and manpower run low).
There is one important nuance: Ukraine should also impose a no-fly zone over the rebel-held zone. This might seem counter-intuitive, given the fact that the Ukrainians have access to air-power and the rebels do not, (and air-power has already granted a significant advantage to Ukrainian forces in several engagements), but once encircled the rebels will only have the option of being supplied by air. If Ukraine declares a no-fly zone, it will gain a number of other advantages in return. First, its weakened air forces will be less exposed to attack and losses, and Ukraine will have to maintain as strong an air force as possible given the threat of a more open attack by Russia. Second, if the air force is not operating over rebel areas, it will be harder for the rebels and Russia to claim attacks on civilian areas are being made by Ukraine from the air (some sort of monitoring of the no-fly zone, perhaps by the OSCE, will also be required). Third: any flights made by Russian military aircraft or even civilian helicopters intended to supply the rebels will be open to attack by Ukraine – if it's in the air, shoot it down. Ukraine's own forces in the area can be supplied by road and rail rather than by air.
The last, but perhaps the most important point is this: to achieve a military victory, the one thing Ukraine must not do is agree to another ceasefire. That would simply allow the rebels to regroup, resupply and reinforce. That would be a disastrous mistake, comparable to Rundstedt's in 1940, which ultimately led to defeat.