Once rock-solid, the support Ukraine has received from its biggest backers for its defense against Russia is showing cracks. Political posturing in places like Poland and Slovakia, where a trade dispute with Ukraine has exacerbated tensions, and Republican reticence in the United States about Washington’s big spending to prop up Ukraine’s military have raised new uncertainties about the West’s commitment to its efforts to expel Russian invaders more than 18 months into the war. U.S. and Polish officials have sought to play down any tiff that some say plays into the hands of Russian President Vladimir Putin.