I'm reading "The Guns of August", and in the chapter "Home before the Leaves fall", the author mentioned 2 instances when German embasssy/legation was "taken over" by American ambassadors after Germany declared war on France and Belgium, respectively.
What is the meaning of this action?
Protecting power is a neutral state that assume responsibility for the interests of belligerents during times of war. It is common for protecting powers to be appointed when two countries break off diplomatic relations with each other. The protecting power is responsible for looking after the sending state's diplomatic property and citizens in the hosting state.
For example, during the Franco-Prussian War (1870-71) the United Kingdom took on the role of protecting power for the French; in the Sino-Japanese War (1894-95) the United States acted as protecting power for both parties; during the Spanish/American conflict of 1898, France and Austria-Hungary assumed a joint protecting power role for the Spanish, etc.
The United States was initially a popular choice for protecting power, going back to its protection of the North German Confederation during the Franco-Prussian War. Between 1914 and 1917, the United States accepted a total of 54 mandates as protecting power. When the United States entered the war on the Allied side in 1917, the American mandates were transferred to smaller neutrals.
Source: Diplomacy under a foreign flag: when nations break relations; Newsom, David D; 1989.