The first thing to know: Long convoys of Russian military trucks, tanks, towed artillery, and other weapons are moving away from Ukraine's Donbas region, headed southwest -- and in at least one case, headed northwest from the Russian-occupied port of Mariupol.

The second thing to know: Bridges, ammunition depots, and rail links are being targeted with growing frequency in Ukraine's southern regions, hit by Ukrainian forces with powerful Western-supplied weapons.

The most important thing to know: Russia's war on Ukraine is about to enter a new phase, if it hasn't already, with the heaviest fighting shifting to a roughly 350-kilometer front line stretching southwest from near Zaporizhzhya to Kherson, paralleling the Dnieper River.

The fight for the city of Kherson alone "could prove decisive in determining the ultimate outcome of the war," according to at least one account.

"The south is key," said Peter Zwack, a retired U.S. Army general who served as defense attache at the U.S. Embassy in Moscow.

"It's more than just a test, from Kyiv's perspective," he told RFE/RL. "The fight in the south is existential. Certainly [the Ukrainians] are refining their forces, bringing in new men, equipment, but first and foremost, it's existential. They've got to get the Russians out of the south now -- before everything hardens, coalesces, they've got to push them out."

Already, Ukrainian officials are pointing to a growing number of villages and towns in the Kherson region that have been recaptured. The region's head, Dmytro Butriy, said on national TV on August 2 that 53 settlements there had been recaptured since the start of the invasion - nine of them over the previous 24 hours.

Ukrainian commanders said that Russian forces were massing in the south in preparation for an assault -- possibly in response to, or anticipation of, Ukraine's counteroffensive.

Ukrainian artillery and long-range rockets have also hit a growing number of targets of strategic importance to Russia in the Kherson region, including bridges, and more recently, a railroad spur that links it to Russian-occupied Crimea.

Counteroffensive In Southern Ukraine

"You all have been reporting on the advances that the Ukrainians are making around Kherson -- and we have seen [them]," a senior U.S. Pentagon official said last week, "and they're not large, giant advances but they are certainly advances against the Russians."

"I don't know if it's a part of a counteroffensive," the official said at a briefing for journalists. "I know they're making offensive gains. And they are doing so, you know, against a Russian force that...looks like they are ill-prepared for it."

First To Fall

In the first weeks after Russia invaded on February 24, much of the fighting was concentrated in the north, near Kyiv, and in the south along the Sea of Azov coast, around Mariupol. Ukraine won in the north, driving Russian forces back across the border. Russia made substantial gains in the south, including seizing Kherson, which had a pre-invasion population of around 300,000 and was the first major Ukrainian city to be taken. Mariupol was captured after a brutal siege.

The focus then shifted east, to the Donbas, where Russia regrouped and then pummeled Ukrainian troops with withering artillery and rocket fire and brutal urban warfare. Ukrainian forces were forced to retreat. Russia seized major Luhansk region cities, before taking an "operational pause."

In the south, however, for weeks, Ukrainian forces have made small, incremental gains in the Kherson region, which straddles the Dnieper. The river has been a natural defensive barrier, and the advances Ukrainian forces have made have been on the northern side, or right bank.

The Antonivsky Bridge across the Dnieper River near the Russian-held city of Kherson has been badly damaged by Ukrainian strikes.

Now, according to military experts, eyewitness reports, social media accounts, and Western intelligence, both Ukraine and Russia are girding for a new concentration of fighting along the Dnieper.

In the Kherson and Zaporizhzhya regions, Russia's southern grouping is built around the 49th Combined Arms Army, along with mechanized and paratroopers brigades as well as some units that have been mobilized from the occupied parts of the Donetsk and Luhansk regions.

Ukraine's military intelligence agency said on August 2 that a new battalion tactical group, which comprises between 800 and 1,000 troops, had deployed to Crimea and would be sent into the Kherson region in the coming days.

Russian positions on the northern bank of the Dnieper were untenable and vulnerable, according to Mykola Byelyeskov, a research fellow at the National Institute for Strategic Studies, a Ukrainian government think tank. That's due in large part to Ukraine's use of U.S.-supplied M142 High Mobility Artillery Rocket Systems, known as HIMARS.

Copyright (c) 2018. RFE/RL, Inc. Republished with the permission of Radio Free Europe/Radio Liberty, Washington DC 20036