In 3–5 sentences, interpret how the government attempted to gain citizens’ cooperation using this war poster.

Answer:
In the context of this poster, "loose talk" means to conversation/gossip and "cost lives" refers to the war currently going on. The exclamation, "Quiet!" warns the public to not gossip about matters that could make the war worse or put the country at risk. Examples of these matters might include war plans or even negative information about the United States government.
The poster above with the phrase "loose talk" connotes the conversation or gossip and "cost lives" is said to be the war that is currently happening.
This is known to be posters that are used to to motivate and influence the American people in a a lot of ways.
Note that the The exclamation, "Quiet!" infers that the public should gossip about war matters so that all could be safe and not to make the war harder or put the country at risk.
Conclusively, The aim is to boast patriotism and also boast public support in times of war.
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