British Foreign Secretary Liz Truss is expected to warn against the “appeasement” of Russian President Vladimir Putin in a speech during a visit to Bosnia-Herzegovina on May 26 and tell Western allies and partners there must be no “backsliding” in support for Ukraine.

“Russia’s aggression cannot be appeased. It must be met with strength,” Truss will say, according to excerpts of the speech to be delivered to Bosnian armed forces at Sarajevo’s Army Hall.

Truss will use the speech to rally allies to deliver more weapons and targeted sanctions against Moscow and argue “we must all learn the lessons of history” in standing up to Putin, ensuring he loses in Ukraine.

“We must not allow a prolonged and increasingly painful conflict to develop in Ukraine…. We must be relentless in ensuring Ukraine prevails through military aid and sanctions. We can’t take our foot off the accelerator now,” she will say.

Britain’s top diplomat will also use the speech to reaffirm her country’s commitment to peace and stability in the Western Balkans “in the face of Russian malign influence.”

Truss is expected to take a step toward this by announcing during the speech her aim to mobilize $100 million of U.K.-backed investment in the Western Balkans by 2025 through an investment partnerships agenda.

“Bosnia and Herzegovina’s future lies in sovereignty and self-determination, in greater partnership with NATO and countries like the U.K.,” Truss will say.

Russia launched its unprovoked invasion of Ukraine on February 24.