There are two ways to answer this.
First, the US government used repressive laws to prevent dissent against the war. These were the Espionage and Sedition Acts. They did things such as prohibiting any criticism of the war effort.
Second, the government used a massive propaganda campaign to actively build support for the war. This campaign was coordinated by the Committee on Public Information (CPI). The CPI put out propaganda both on a large and small scale. They organized tens of thousands of "Four-Minute Men" who would go around and give pro-war speeches to various groups. On a larger scale, they placed propaganda ads in newspapers and magazines and also did things like putting out stories about German atrocities that were meant to get the people to be more anti-German.
Therefore, the answer could be "repressive laws," "propaganda" or both.