Tensions between Ukraine and Russia escalated on Monday after Russia captured Ukrainian ships, prior to a gun fire. Russia then closed the Kerch Strait (waterway for ships in or out of Ukraine’s eastern cities).
This act put Ukraine on alert, the Ukrainian Parliament approved martial law imposition in Ukraine’s coastal regions and those bordering Russia and Transnistria.

Events that led to such an atmosphere
Earlier in 2014 Russia annexed Crimea. This annexation has been condemned by world leaders and is still not officially recognized by the United Nations.
The Kerch Strait connects the Sea of Azov with the Black Sea, and formed by the coastlines of Russia and Crimea. It is the only waterway for ships in or out of Ukraine’s eastern port cities. While Ukraine and Russia have rights to use the straits, Russia controls both ends of the strait following the Crimean annexation.
In May 2018, Russia constructed a 19 kilometres long bridge along the strait, allowing direct land connection between Crimea and Moscow.
The international community in recent times is against Russia. So naturally it becomes easy for Ukraine to gain international sympathy, even if it is wrong here.