BERLIN—German Chancellor Angela Merkel will open her traditional New Year’s address with a pledge to stand firm against Russia’s actions in eastern Ukraine, saying Europe won’t accept Russia’s strong-arming in the region.

“It’s completely beyond question that we want security in Europe together with Russia, not against Russia. But it’s also completely beyond question that Europe cannot and will not accept an alleged law of the strongest that disregards international law,” Ms. Merkel will tell viewers according to a copy of...