Yep, illegal. The constitution does say separation of church and state.

Yay!!!! I love this point made during the proceedings "The monument here has the primary effect of endorsing religion and excessively entangles the government in religion. The Latin cross is the core symbol of Christianity. And here, it is 40 feet tall; prominently displayed in the center of one of the busiest intersections in Prince George’s County, Maryland; and maintained with thousands of dollars in government funds. Therefore, we hold that the purported war memorial breaches the “wall of separation between Church and State.” Truly our taxpayer dollars should not be used to support a monument like this. Reading this has made my day - SUPER HAPPY!