Ukraine pressed ahead 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 aims to 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, which President Biden said was creating a “real dilemma” for the Russian government.

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 death and ruin.