Passage: Rumors had reached the Lithuanian government to the effect that Germany had specific plans to take over Klaipėda. On 12 March Foreign Minister Urbšys represented Lithuania at the coronation of Pope Pius XII in Rome. On his return to Lithuania he stopped in Berlin with the hope of clarifying the growing rumors. On 20 March Ribbentrop agreed to meet with Urbšys, but not with Kazys Škirpa, who was asked to wait in another room. The conversation lasted for about 40 minutes. Ribbentrop demanded the return of Klaipėda to Germany and threatened military action. Urbšys relayed the verbal ultimatum to the Lithuanian government. Because the ultimatum was never set down in writing and did not include a formal deadline, some historians have downplayed its import, describing it as a "set of demands" rather than as an ultimatum. However, it was made clear that force would be used should Lithuania resist, and it was warned not to seek help from other nations. While a clear deadline was not given, Lithuania was told to make a speedy decision and that any clashes or German casualties would inevitably provoke a response from the German military.Lithuania secretly informed the signatories of the Klaipėda Convention about these demands, since technically Lithuania could not transfer Klaipėda without the approval of the signatories. Italy and Japan supported Germany in the matter, while the United Kingdom and France expressed sympathy for Lithuania but chose not to offer any material assistance. They followed a well-publicized policy of appeasing Hitler. The UK treated the issue in the same way as it had treated the Sudeten Crisis and made no plans to assist Lithuania or the other Baltic States if they were attacked by Germany. The Soviet Union, while supporting Lithuania in principle, did not wish to disrupt its relations with Germany at that point, since it was contemplating an alliance with the Nazis. Without any material international support Lithuania had no choice but to accept the ultimatum. Lithuanian diplomacy...

Question: In order to get back what they lost ribbentrop threatened what?
military action