Line
The use of diagonal lines is a way to show drama. In Washington Crossing the Delaware there are many diagonal lines. For example, many of the people in the boat with Washington are leaning off the edge or trying to keep their balance while riding in the boat that is sailing through the waves, so the sides of their bodies are diagonal lines. The implied dram could be that they are on their way to fight a battle and they are unsure of the outcome and level of violence.
Diagonal lines could also be used like arrows; they point toward the subject of the painting or something else the artist wants the viewer to notice. The oars that are moving the boats are diagonal lines that are pointing toward Washington. The flag also has diagonal lines that are pointing toward Washington. Since the flag is next to Washington, the oars are drawing attention to it as well, letting the viewer know that it is an American battle.