By John Chalmers and Pavel Polityuk BRUSSELS/LVIV/MARIUPOL, Ukraine (Reuters) – Western leaders agreed on Wednesday to strengthen their forces on Europe’s eastern flank, increase military aid to Ukraine and tighten sanctions on Russia whose invasion and bombardment of its neighbour entered a second month. At an unprecedented triple summit, transatlantic alliance NATO, G7 rich nations and European leaders addressed the continent’s biggest conflict since the 1990s Balkans wars. NATO announced new battle groups for four Eastern European nations, while the United States and Britain expanded sancti…