KYIV — Many U.S.-made satellite-guided munitions in Ukraine have failed to withstand Russian jamming technology, prompting Kyiv to stop using certain types of Western-provided armaments after effectiveness rates plummeted, according to senior Ukrainian military officials and confidential internal Ukrainian assessments obtained by The Washington Post. Russia’s jamming of the guidance systems of modern Western weapons, including Excalibur GPS-guided artillery shells and the High Mobility Artillery Rocket System, or HIMARS, which can fire some U.S.-made rockets with a range of up to 50 miles, has eroded Ukraine’s ability to defend its territory and has left officials in Kyiv urgently seeking help from the Pentagon to obtain upgrades from arms manufacturers. Russia’s ability to combat the high-tech munitions has far-reaching implications for Ukraine and its Western supporters — potentially providing a blueprint for adversaries such as China and Iran — and it is a key reason Moscow’s forces have regained the initiative and are advancing on the battlefield.