Rav Avigdor Miller zt”l held that Jews must behave patriotically and even display an American flag outside their homes on July 4. I’m guessing that he would have held to show respect to U.S. veterans of previous wars. Without their sacrifice, we might be speaking German (if we’d be alive…)
Israeli Yom Hazikaron: yup, that day when the state honors its dead with gentile ceremonies such as blowing sirens, laying wreaths, placing Israeli flags on the graves of the fallen soldiers. Oh yeah, let’s not forget saying Kaddish on an arbitrary day, not the yahrzeit of the fallen soldier. Maybe it doesn’t hurt the neshama of the niftar (saying Kaddish on the wrong day) but it sure doesn’t help. Saying Tehillim le’iluy nishmasam probably doesn’t hurt, even if it’s not the yahrzeit.
Bottom line: Memorial Day in the U.S. doesn’t conflict with any halachos or Jewish practices. It’s made up by the gentiles, and Jews can and should show respect. Israeli Yom Hazikaron, however, contravenes Jewish practices, so why follow the dictates of a bunch of atheists who decided to create a mass-yahrzeit day? (Same question applies to 5 Iyar too, not just 4 Iyar.) The authentic Jewish way of memorializing neshamos is a benefit to the fallen. The idiotic gentile ceremonies that are conducted here are of no benefit to them. Maybe even causes them distress.