Western powers chose not to intervene in Bosnia for four years following the breakup of Yugoslavia in the early 1990s.

During the 1992-1995 war, more than 100,000 people were killed, resulting in mass murder on a scale not seen in Europe since World War II.

Now, a quarter-century after the peace treaty that ended the war, those powers are confronted with the possibility of deadly sectarian fighting.

The foreign secretary will also announce plans to strengthen “security and economic partnership with Bosnia in the face of secessionists and Russian attempts to influence and destabilize,” according to a statement from the Foreign Office. Among these plans is a bid to “mobilize $100 million in UK-backed investment in the Western Balkans by 2025.”

Truss was scheduled to address military personnel at Sarajevo’s Army Hall, warning that Russia’s aggression “cannot be appeased” and that Bosnia’s “future lies in sovereignty and self-determination, in greater partnership with Nato and countries like the UK.”

Following Vladimir Putin’s invasion of Ukraine, Sarajevo last month urged Nato to expedite its bid for membership in order to maintain regional security.

Sifet Podzic, Bosnia’s Defense Minister, told Al Jazeera that his country was taking part in the Membership Action Plan, which he described as “the final step before gaining [Nato] membership.” But, he warned, if the alliance insists that Bosnia “satisfies all criteria,” gaining membership “will take a while,” because “we will need a lot of time to modernize our army.”

The Bosnian War began when Bosnia and Herzegovina declared independence from several republics of the former Yugoslavia. The General Framework Agreement for Peace in Bosnia and Herzegovina ended the conflict after three and a half years of bloodshed.

The International Criminal Tribunal for the Former Yugoslavia had convicted 45 Serbs, 12 Croats, and four Bosnians of war crimes committed during the war by early 2008.

Thousands of Bosnians and Croats died in Serb-run concentration camps at locations such as Omarska and Trnopolje. The most notorious act of genocide, however, was the Srebrenica massacre, which killed 8,000 Bosnian men and boys.

The peace treaty, known as the Dayton Accords, saw the majority Muslim Bosnians and Serb separatists fighting under the banner of the Republika Srpska agree to a single sovereign state.

However, in what The Washington Post described as a “complicated compromise,” this state was divided into two parts: the predominantly Serb-populated Republika Srpska and the predominantly Croat-Bosnian Federation of Bosnia and Herzegovina.

The agreement “reestablished Bosnia as a unified state and granted the right of return to victims of ethnic cleansing,” the paper continued, contradicting the “wishes of Serb and Croat ultra-nationalists.”

However, it also “adopted ethnic federal structures that recognize Republika Srpska as a political entity with self-governing rights,” which Bosnians objected to.

Since then, the uneasy peace has been maintained. However, in November, UN High Representative Christian Schmidt warned that threats by Milorad Dodik, the Serb member of the presidency as part of a triumvirate power-sharing agreement, to “pull out of state-level institutions” amounted to “secession.”

The suggestion that Serbs form their own military “endangers not only the country’s and the region’s peace and stability, but – if unanswered by the international community – could lead to the undoing of the [Dayton] agreement itself,” Schmidt wrote in his first report as Bosnia’s overseer.

A splintering of the military “into two or more armies” would necessitate a “reassessment of the level of international military presence,” he added. “A failure to respond to the current situation would jeopardize the [Dayton Accord], while instability in Bosnia and Herzegovina would have wider regional implications.”

Schmidt’s report came just days after Bosnian Serb police conducted a “anti-terrorist” drill just outside Sarajevo. According to Al Jazeera, the move was “seen by many as another provocation by the Serb separatist leadership.”