evidence: Mr Biden's surprise visit to the Ukrainian capital on Monday was the first time in modern history that a US leader has visited a war zone outside the shield of the US military. [...] While Mr Biden was in Kyiv, US surveillance planes, including an E-3 Sentry airborne radar and an electronic RC-135W Rivet Joint aircraft, were keeping watch over Kyiv from Polish airspace. [...] It was only then that the first images of Mr Biden in the capital popped up on Ukrainian social media and the secret visit became global news.