The limitations of American diplomacy — at the border between Russia and Ukraine, and at the Olympics in Beijing.Read more:According to U.S. intelligence and The Post’s reporting, Russia is planning to move up to 175,000 troops to its border with Ukraine — plans that have the international community concerned. On a call with Russian President Vladimir Putin on Tuesday, President Biden threatened economic sanctions and other measures if the Kremlin were to escalate the situation and invade Ukraine. Shane Harris reports on Putin’s plans, and on how difficult it is to deter a country like Russia.Plus, the United States’ diplomatic boycott of the upcoming Beijing Winter Olympics. Rick Maese reports on the pointed snub in protest of China’s human rights abuses.