The attitude of Europeans and other observers was one not of fear of the United States, but a combination of lack of respect and a conviction that Americans were essentially hypocrites to talk about ideals then aim at expanding their land holdings.
This conviction developed, in part, out of American propaganda and publicity. The Americans did a great deal of talking and writing about liberty, but at the same time, they expanded the idea of Manifest Destiny. It was their destiny to expand across North America. The people poised in the way of that expansion, were aware of this, especially the Mexicans.
Mexicans were torn between two conflicting attitudes about the United States. One was an attitude of admiration, the other was an attitude of fear that the Americans would try to detach border territories from Mexico's lands.