What event involved pro-Russian forces attempting to annex a territory in Ukraine?

The event that involved pro-Russian forces attempting to annex a territory in Ukraine is the invasion of Crimea. This occurred in early 2014 when Russia deployed troops into the Crimean Peninsula following Ukraine’s Euromaidan protests and the ousting of President Yanukovych. The military presence was justified by Russia under the pretext of protecting ethnic Russians and Russian speakers in Crimea. Subsequently, a controversial referendum was held in which the Crimean authorities claimed a majority of voters supported joining Russia, leading to Russia’s formal annexation of Crimea. This act was widely condemned by the international community as a violation of Ukraine's sovereignty and territorial integrity.

The other events and options presented do not accurately pertain to a specific attempt at annexation by pro-Russian forces in Ukraine. The occupation of Ukraine is a broader term and does not focus exclusively on the Crimea conflict. The annexation of Latvia refers to a different historical context, particularly during World War II, which is unrelated to the situation in Ukraine. The protests in Kiev, while significant events leading up to the conflict, represent internal dynamics in Ukraine rather than an annexation effort by pro-Russian forces.