Q:

When did Yeats enter into "Virgil's territory"?

In the definition of "divagate," several dictionaries, like this one, give the following example sentence:

Yeats divagated into Virgil's territory only once.

What instance of Yeats' writing does this refer to?

A:

The original source of the quote seems to be Virgil in a Cultural Tradition: Essays to Celebrate the Bimillenium, edited by 
Richard Andrew Cardwell and Janet Hamilton. Google books shows that this book contains the quote

Yeats divagated into Virgil's territory only when, in 1915, he wrote Per Amica Silentia Lunae, going for that title to Aeneid 2, 255, A Tenedo tacitae per amica silentia lunae.

However, referring to the 1918 poem Shepherd and Goatherd that he wrote in memory of Major Robert Gregory, Yeats says

... my poem about Robert, a pastoral, modelled on what Virgil wrote for some friend of his ..."

So it seems the dictionary quote is wrong, and Yeats divagated into Virgil's territory at least twice.

