WASHINGTON – The U.S. and Israel are embarking on the largest exercise in their long military relationship, against a backdrop of tension with Iran and sharp rhetoric in the American presidential election.

An air defense drill planned for late this month will involve more than 3,500 Americans and 1,000 Israelis, practicing their ability to work together against a range of threats facing the main U.S. ally in the Mideast. Officials briefing reporters on Wednesday would not specify the exact date for security reasons.

American forces have started arriving in Israel and others will participate from positions around Europe and the Mediterranean in what the military is calling Austere Challenge 2012, U.S. Air Force Lt. Gen. Craig Franklin said via telephone from Germany.