Ukraine was pressing on with its assault inside Russian territory on Tuesday, a week into the biggest foreign incursion into the country since World War II.

The cross-border attack caught Russia by surprise and signified a shift in tactics for Kyiv, more than two years after Moscow’s troops poured across Ukraine’s border in a full-scale invasion.

The rapid advance by Ukrainian forces has been an embarrassment for the Kremlin and could alter the narrative of the war at a time when Kyiv’s forces are stretched thin on the front lines of their own country.

Here’s what to know about Ukraine’s cross-border operation in Russia.

What happened?

Ukrainian troops and armored vehicles stormed into the Kursk region of western Russia the morning of Aug. 6, punching through border defenses and seizing several settlements in heavy fighting that left a trail of dead and ruin.

The assault opened a new front in the 30-month war and did not just catch Russia off guard: Some Ukrainian soldiers and U.S. officials also said they lacked advance notice.

Russia’s top general estimated that Ukraine had deployed 1,000 soldiers for the incursion, while U.S. officials said Ukraine had sent several thousand. Military analysts have said the attack involved elements of at least four brigades in a rare example of a successful operation involving support from artillery, air defenses and electronic warfare. That translated into quick advances on the ground.