Weird to see DL be the follower, not the leader, into a new market.
Actually, it seems over the last 5 years or so United has been the leader of the big three in opening up new international markets as well as connecting additional points to existing international destinations
Yes. United I think flies more international routes of the US3 (pre-pandemic). When UA and CO merged, they combined rather large international footprints. In terms of new P2P international routes, UA has added quite a number in recent years, including some I've listed below (partial list from memory), not all of which are currently operating of course (they are not in any particular order):
EWR-PMO (never started, but would have launched in June 2020)
EWR-NCE (announced for Summer 2020 but didn't launch)
EWR-JNB (announced, but didn't launch, I think)
SFO-BLR (announced, not started)
SFO-CDG (launched a while back, not operating now)
IAD-ATH (announced for Summer 2021)
EWR-IST (operated 2012-2013, then cancelled)
EWR-EZE* (operated on and off, starting in 2012, cancelled in 2013, and then resumed in 2017 and then suspended in 2019).
This route has moved around a lot. The origins of it are JFK-EZE, which then moved to IAD-EZE and then moved to EWR-EZE.
LAX-SIN (started, then suspended, and transferred to SFO for 2 x daily until the pandemic)